LibWeb/Fetch: Port infrastructure to new String

19 files changed, 141 insertions, 140 deletions

diff --git a/Userland/Libraries/LibWeb/Fetch/Body.cpp b/Userland/Libraries/LibWeb/Fetch/Body.cpp
index 8290cf954b..c309beec61 100644
--- a/Userland/Libraries/LibWeb/Fetch/Body.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Body.cpp
@@ -158,7 +158,7 @@ JS::NonnullGCPtr<JS::Promise> consume_body(JS::Realm& realm, BodyMixin const& ob
     // 3. If object’s body is non-null, then set promise to the result of fully reading body as promise given object’s body.
     auto const& body = object.body_impl();
     if (body.has_value())
-        promise = body->fully_read_as_promise();
+        promise = body->fully_read_as_promise().release_value_but_fixme_should_propagate_errors();
 
     // 4. Let steps be to return the result of package data with the first argument given, type, and object’s MIME type.
     auto steps = [&vm, &realm, &object, type](JS::Value value) -> WebIDL::ExceptionOr<JS::Value> {
diff --git a/Userland/Libraries/LibWeb/Fetch/FetchMethod.cpp b/Userland/Libraries/LibWeb/Fetch/FetchMethod.cpp
index e42cd55a52..1fd1e04b1e 100644
--- a/Userland/Libraries/LibWeb/Fetch/FetchMethod.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/FetchMethod.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -99,8 +99,8 @@ JS::NonnullGCPtr<JS::Promise> fetch_impl(JS::VM& vm, RequestInfo const& input, R
 
         // 3. If response is a network error, then reject p with a TypeError and abort these steps.
         if (response->is_network_error()) {
-            auto message = response->network_error_message().value_or("Response is a network error"sv);
-            WebIDL::reject_promise(vm, promise_capability, JS::TypeError::create(relevant_realm, message).release_allocated_value_but_fixme_should_propagate_errors());
+            auto message = response->network_error_message().value_or("Response is a network error"_string.release_value_but_fixme_should_propagate_errors());
+            WebIDL::reject_promise(vm, promise_capability, JS::TypeError::create(relevant_realm, move(message)));
             return;
         }
 
diff --git a/Userland/Libraries/LibWeb/Fetch/Fetching/Checks.cpp b/Userland/Libraries/LibWeb/Fetch/Fetching/Checks.cpp
index f019327a29..c2492e789e 100644
--- a/Userland/Libraries/LibWeb/Fetch/Fetching/Checks.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Fetching/Checks.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -56,11 +56,11 @@ ErrorOr<bool> tao_check(Infrastructure::Request const& request, Infrastructure::
     auto values = TRY(response.header_list()->get_decode_and_split("Timing-Allow-Origin"sv.bytes()));
 
     // 3. If values contains "*", then return success.
-    if (values.has_value() && values->contains_slow("*"sv))
+    if (values.has_value() && values->contains_slow("*"_short_string))
         return true;
 
     // 4. If values contains the result of serializing a request origin with request, then return success.
-    if (values.has_value() && values->contains_slow(request.serialize_origin()))
+    if (values.has_value() && values->contains_slow(TRY(request.serialize_origin())))
         return true;
 
     // 5. If request’s mode is "navigate" and request’s current URL’s origin is not same origin with request’s origin, then return failure.
diff --git a/Userland/Libraries/LibWeb/Fetch/Fetching/Fetching.cpp b/Userland/Libraries/LibWeb/Fetch/Fetching/Fetching.cpp
index 2c1829aee8..23eb7cf540 100644
--- a/Userland/Libraries/LibWeb/Fetch/Fetching/Fetching.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Fetching/Fetching.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  * Copyright (c) 2023, Luke Wilde <lukew@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
@@ -214,7 +214,7 @@ WebIDL::ExceptionOr<Optional<JS::NonnullGCPtr<PendingResponse>>> main_fetch(JS::
     // 3. If request’s local-URLs-only flag is set and request’s current URL is not local, then set response to a
     //    network error.
     if (request->local_urls_only() && !Infrastructure::is_local_url(request->current_url()))
-        response = Infrastructure::Response::network_error(vm, "Request with 'local-URLs-only' flag must have a local URL"sv);
+        response = Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request with 'local-URLs-only' flag must have a local URL"_string));
 
     // FIXME: 4. Run report Content Security Policy violations for request.
     // FIXME: 5. Upgrade request to a potentially trustworthy URL, if appropriate.
@@ -225,7 +225,7 @@ WebIDL::ExceptionOr<Optional<JS::NonnullGCPtr<PendingResponse>>> main_fetch(JS::
         || false // FIXME: "should fetching request be blocked as mixed content"
         || false // FIXME: "should request be blocked by Content Security Policy returns blocked"
     ) {
-        response = Infrastructure::Response::network_error(vm, "Request was blocked"sv);
+        response = Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request was blocked"_string));
     }
 
     // 7. If request’s referrer policy is the empty string, then set request’s referrer policy to request’s policy
@@ -300,13 +300,13 @@ WebIDL::ExceptionOr<Optional<JS::NonnullGCPtr<PendingResponse>>> main_fetch(JS::
         // -> request’s mode is "same-origin"
         else if (request->mode() == Infrastructure::Request::Mode::SameOrigin) {
             // Return a network error.
-            return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request with 'same-origin' mode must have same URL and request origin"sv));
+            return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request with 'same-origin' mode must have same URL and request origin"_string)));
         }
         // -> request’s mode is "no-cors"
         else if (request->mode() == Infrastructure::Request::Mode::NoCORS) {
             // 1. If request’s redirect mode is not "follow", then return a network error.
             if (request->redirect_mode() != Infrastructure::Request::RedirectMode::Follow)
-                return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request with 'no-cors' mode must have redirect mode set to 'follow'"sv));
+                return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request with 'no-cors' mode must have redirect mode set to 'follow'"_string)));
 
             // 2. Set request’s response tainting to "opaque".
             request->set_response_tainting(Infrastructure::Request::ResponseTainting::Opaque);
@@ -320,7 +320,7 @@ WebIDL::ExceptionOr<Optional<JS::NonnullGCPtr<PendingResponse>>> main_fetch(JS::
             VERIFY(request->mode() == Infrastructure::Request::Mode::CORS);
 
             // Return a network error.
-            return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request with 'cors' mode must have URL with HTTP or HTTPS scheme"sv));
+            return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request with 'cors' mode must have URL with HTTP or HTTPS scheme"_string)));
         }
         // -> request’s use-CORS-preflight flag is set
         // -> request’s unsafe-request flag is set and either request’s method is not a CORS-safelisted method or
@@ -451,7 +451,7 @@ WebIDL::ExceptionOr<Optional<JS::NonnullGCPtr<PendingResponse>>> main_fetch(JS::
                     // - should internalResponse to request be blocked due to nosniff
                     || TRY_OR_IGNORE(Infrastructure::should_response_to_request_be_blocked_due_to_nosniff(internal_response, request)) == Infrastructure::RequestOrResponseBlocking::Blocked)) {
                 // then set response and internalResponse to a network error.
-                response = internal_response = Infrastructure::Response::network_error(vm, "Response was blocked"sv);
+                response = internal_response = Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("Response was blocked"_string));
             }
 
             // 19. If response’s type is "opaque", internalResponse’s status is 206, internalResponse’s range-requested
@@ -464,7 +464,7 @@ WebIDL::ExceptionOr<Optional<JS::NonnullGCPtr<PendingResponse>>> main_fetch(JS::
                 && internal_response->status() == 206
                 && internal_response->range_requested()
                 && !request->header_list()->contains("Range"sv.bytes())) {
-                response = internal_response = Infrastructure::Response::network_error(vm, "Response has status 206 and 'range-requested' flag set, but request has no 'Range' header"sv);
+                response = internal_response = Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("Response has status 206 and 'range-requested' flag set, but request has no 'Range' header"_string));
             }
 
             // 20. If response is not a network error and either request’s method is `HEAD` or `CONNECT`, or
@@ -478,8 +478,8 @@ WebIDL::ExceptionOr<Optional<JS::NonnullGCPtr<PendingResponse>>> main_fetch(JS::
             if (!request->integrity_metadata().is_empty()) {
                 // 1. Let processBodyError be this step: run fetch response handover given fetchParams and a network
                 //    error.
-                auto process_body_error = [&] {
-                    return fetch_response_handover(realm, fetch_params, Infrastructure::Response::network_error(vm, "Response body could not be processed"sv));
+                auto process_body_error = [&]() -> WebIDL::ExceptionOr<void> {
+                    return fetch_response_handover(realm, fetch_params, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Response body could not be processed"_string)));
                 };
 
                 // 2. If response’s body is null, then run processBodyError and abort these steps.
@@ -668,7 +668,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> scheme_fetch(JS::Realm& r
 
     // 1. If fetchParams is canceled, then return the appropriate network error for fetchParams.
     if (fetch_params.is_canceled())
-        return PendingResponse::create(vm, fetch_params.request(), Infrastructure::Response::appropriate_network_error(vm, fetch_params));
+        return PendingResponse::create(vm, fetch_params.request(), TRY_OR_THROW_OOM(vm, Infrastructure::Response::appropriate_network_error(vm, fetch_params)));
 
     // 2. Let request be fetchParams’s request.
     auto request = fetch_params.request();
@@ -693,7 +693,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> scheme_fetch(JS::Realm& r
     // -> "blob"
     else if (request->current_url().scheme() == "blob"sv) {
         // FIXME: Support 'blob://' URLs
-        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request has 'blob:' URL which is currently unsupported"sv));
+        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request has 'blob:' URL which is currently unsupported"_string)));
     }
     // -> "data"
     else if (request->current_url().scheme() == "data"sv) {
@@ -705,7 +705,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> scheme_fetch(JS::Realm& r
 
         // 2. If dataURLStruct is failure, then return a network error.
         if (data_or_error.is_error())
-            return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request has invalid base64 'data:' URL"sv));
+            return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request has invalid base64 'data:' URL"_string)));
 
         // 3. Let mimeType be dataURLStruct’s MIME type, serialized.
         auto const& mime_type = url.data_mime_type();
@@ -724,7 +724,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> scheme_fetch(JS::Realm& r
         // For now, unfortunate as it is, file: URLs are left as an exercise for the reader.
         // When in doubt, return a network error.
         // FIXME: Support 'file://' URLs
-        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request has 'file:' URL which is currently unsupported"sv));
+        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request has 'file:' URL which is currently unsupported"_string)));
     }
     // -> HTTP(S) scheme
     else if (Infrastructure::is_http_or_https_scheme(request->current_url().scheme())) {
@@ -734,9 +734,9 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> scheme_fetch(JS::Realm& r
 
     // 4. Return a network error.
     auto message = request->current_url().scheme() == "about"sv
-        ? "Request has invalid 'about:' URL, only 'about:blank' can be fetched"sv
-        : "Request URL has invalid scheme, must be one of 'about', 'blob', 'data', 'file', 'http', or 'https'"sv;
-    return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, message));
+        ? TRY_OR_THROW_OOM(vm, "Request has invalid 'about:' URL, only 'about:blank' can be fetched"_string)
+        : TRY_OR_THROW_OOM(vm, "Request URL has invalid scheme, must be one of 'about', 'blob', 'data', 'file', 'http', or 'https'"_string);
+    return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, move(message)));
 }
 
 // https://fetch.spec.whatwg.org/#concept-http-fetch
@@ -806,7 +806,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_fetch(JS::Realm& rea
                 // - request’s redirect mode is not "follow" and response’s URL list has more than one item.
                 || (request->redirect_mode() != Infrastructure::Request::RedirectMode::Follow && response->url_list().size() > 1)) {
                 // then return a network error.
-                return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Invalid request/response state combination"sv));
+                return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Invalid request/response state combination"_string)));
             }
         }
     }
@@ -883,7 +883,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_fetch(JS::Realm& rea
             //       a service worker for that matter, it is applied here.
             if (request->response_tainting() == Infrastructure::Request::ResponseTainting::CORS
                 && !TRY_OR_IGNORE(cors_check(request, *response))) {
-                returned_pending_response->resolve(Infrastructure::Response::network_error(vm, "Request with 'cors' response tainting failed CORS check"sv));
+                returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("Request with 'cors' response tainting failed CORS check"_string)));
                 return;
             }
 
@@ -901,7 +901,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_fetch(JS::Realm& rea
         if ((request->response_tainting() == Infrastructure::Request::ResponseTainting::Opaque || response->type() == Infrastructure::Response::Type::Opaque)
             && false // FIXME: "and the cross-origin resource policy check with request’s origin, request’s client, request’s destination, and actualResponse returns blocked"
         ) {
-            returned_pending_response->resolve(Infrastructure::Response::network_error(vm, "Response was blocked by cross-origin resource policy check"sv));
+            returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("Response was blocked by cross-origin resource policy check"_string)));
             return;
         }
 
@@ -918,7 +918,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_fetch(JS::Realm& rea
             // -> "error"
             case Infrastructure::Request::RedirectMode::Error:
                 // Set response to a network error.
-                response = Infrastructure::Response::network_error(vm, "Request with 'error' redirect mode received redirect response"sv);
+                response = Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("Request with 'error' redirect mode received redirect response"_string));
                 break;
             // -> "manual"
             case Infrastructure::Request::RedirectMode::Manual:
@@ -974,7 +974,9 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_redirect_fetch(JS::R
         : static_cast<Infrastructure::FilteredResponse const&>(response).internal_response();
 
     // 3. Let locationURL be actualResponse’s location URL given request’s current URL’s fragment.
-    auto location_url_or_error = actual_response->location_url(request->current_url().fragment());
+    auto const& fragment = request->current_url().fragment();
+    auto fragment_string = fragment.is_null() ? Optional<String> {} : TRY_OR_THROW_OOM(vm, String::from_deprecated_string(fragment));
+    auto location_url_or_error = actual_response->location_url(fragment_string);
 
     // 4. If locationURL is null, then return response.
     if (!location_url_or_error.is_error() && !location_url_or_error.value().has_value())
@@ -982,17 +984,17 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_redirect_fetch(JS::R
 
     // 5. If locationURL is failure, then return a network error.
     if (location_url_or_error.is_error())
-        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request redirect URL is invalid"sv));
+        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request redirect URL is invalid"_string)));
 
     auto location_url = location_url_or_error.release_value().release_value();
 
     // 6. If locationURL’s scheme is not an HTTP(S) scheme, then return a network error.
     if (!Infrastructure::is_http_or_https_scheme(location_url.scheme()))
-        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request redirect URL must have HTTP or HTTPS scheme"sv));
+        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request redirect URL must have HTTP or HTTPS scheme"_string)));
 
     // 7. If request’s redirect count is 20, then return a network error.
     if (request->redirect_count() == 20)
-        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request has reached maximum redirect count of 20"sv));
+        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request has reached maximum redirect count of 20"_string)));
 
     // 8. Increase request’s redirect count by 1.
     request->set_redirect_count(request->redirect_count() + 1);
@@ -1003,20 +1005,20 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_redirect_fetch(JS::R
         && location_url.includes_credentials()
         && request->origin().has<HTML::Origin>()
         && !request->origin().get<HTML::Origin>().is_same_origin(URL::url_origin(location_url))) {
-        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request with 'cors' mode and different URL and request origin must not include credentials in redirect URL"sv));
+        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request with 'cors' mode and different URL and request origin must not include credentials in redirect URL"_string)));
     }
 
     // 10. If request’s response tainting is "cors" and locationURL includes credentials, then return a network error.
     // NOTE: This catches a cross-origin resource redirecting to a same-origin URL.
     if (request->response_tainting() == Infrastructure::Request::ResponseTainting::CORS && location_url.includes_credentials())
-        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request with 'cors' response tainting must not include credentials in redirect URL"sv));
+        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request with 'cors' response tainting must not include credentials in redirect URL"_string)));
 
     // 11. If actualResponse’s status is not 303, request’s body is non-null, and request’s body’s source is null, then
     //     return a network error.
     if (actual_response->status() != 303
         && !request->body().has<Empty>()
         && request->body().get<Infrastructure::Body>().source().has<Empty>()) {
-        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request has body but no body source"sv));
+        return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request has body but no body source"_string)));
     }
 
     // 12. If one of the following is true
@@ -1191,7 +1193,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
         // 8. If contentLength is non-null, then set contentLengthHeaderValue to contentLength, serialized and
         //    isomorphic encoded.
         if (content_length.has_value())
-            content_length_header_value = MUST(ByteBuffer::copy(DeprecatedString::number(*content_length).bytes()));
+            content_length_header_value = MUST(ByteBuffer::copy(TRY_OR_THROW_OOM(vm, String::number(*content_length)).bytes()));
 
         // 9. If contentLengthHeaderValue is non-null, then append (`Content-Length`, contentLengthHeaderValue) to
         //    httpRequest’s header list.
@@ -1324,7 +1326,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
             // 2. If httpRequest’s header list does not contain `Authorization`, then:
             if (!http_request->header_list()->contains("Authorization"sv.bytes())) {
                 // 1. Let authorizationValue be null.
-                auto authorization_value = Optional<DeprecatedString> {};
+                auto authorization_value = Optional<String> {};
 
                 // 2. If there’s an authentication entry for httpRequest and either httpRequest’s use-URL-credentials
                 //    flag is unset or httpRequest’s current URL does not include credentials, then set
@@ -1337,8 +1339,8 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
                 //    true, set authorizationValue to httpRequest’s current URL, converted to an `Authorization` value.
                 else if (http_request->current_url().includes_credentials() && is_authentication_fetch == IsAuthenticationFetch::Yes) {
                     auto const& url = http_request->current_url();
-                    auto payload = DeprecatedString::formatted("{}:{}", url.username(), url.password());
-                    authorization_value = TRY_OR_THROW_OOM(vm, encode_base64(payload.bytes())).to_deprecated_string();
+                    auto payload = TRY_OR_THROW_OOM(vm, String::formatted("{}:{}", url.username(), url.password()));
+                    authorization_value = TRY_OR_THROW_OOM(vm, encode_base64(payload.bytes()));
                 }
 
                 // 4. If authorizationValue is non-null, then append (`Authorization`, authorizationValue) to
@@ -1378,7 +1380,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
 
     // 9. If aborted, then return the appropriate network error for fetchParams.
     if (aborted)
-        return PendingResponse::create(vm, request, Infrastructure::Response::appropriate_network_error(vm, fetch_params));
+        return PendingResponse::create(vm, request, TRY_OR_THROW_OOM(vm, Infrastructure::Response::appropriate_network_error(vm, fetch_params)));
 
     JS::GCPtr<PendingResponse> pending_forward_response;
 
@@ -1386,7 +1388,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
     if (!response) {
         // 1. If httpRequest’s cache mode is "only-if-cached", then return a network error.
         if (http_request->cache_mode() == Infrastructure::Request::CacheMode::OnlyIfCached)
-            return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, "Request with 'only-if-cached' cache mode doesn't have a cached response"sv));
+            return PendingResponse::create(vm, request, Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Request with 'only-if-cached' cache mode doesn't have a cached response"_string)));
 
         // 2. Let forwardResponse be the result of running HTTP-network fetch given httpFetchParams, includeCredentials,
         //    and isNewConnectionFetch.
@@ -1465,7 +1467,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
             if (!request->body().has<Empty>()) {
                 // 1. If request’s body’s source is null, then return a network error.
                 if (request->body().get<Infrastructure::Body>().source().has<Empty>()) {
-                    returned_pending_response->resolve(Infrastructure::Response::network_error(vm, "Request has body but no body source"sv));
+                    returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("Request has body but no body source"_string)));
                     return;
                 }
 
@@ -1483,7 +1485,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
             if (!request->use_url_credentials() || is_authentication_fetch == IsAuthenticationFetch::Yes) {
                 // 1. If fetchParams is canceled, then return the appropriate network error for fetchParams.
                 if (fetch_params.is_canceled()) {
-                    returned_pending_response->resolve(Infrastructure::Response::appropriate_network_error(vm, fetch_params));
+                    returned_pending_response->resolve(TRY_OR_IGNORE(Infrastructure::Response::appropriate_network_error(vm, fetch_params)));
                     return;
                 }
 
@@ -1511,7 +1513,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
                 // 1. If request’s window is "no-window", then return a network error.
                 if (request->window().has<Infrastructure::Request::Window>()
                     && request->window().get<Infrastructure::Request::Window>() == Infrastructure::Request::Window::NoWindow) {
-                    returned_pending_response->resolve(Infrastructure::Response::network_error(vm, "Request requires proxy authentication but has 'no-window' set"sv));
+                    returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("Request requires proxy authentication but has 'no-window' set"_string)));
                     return;
                 }
 
@@ -1520,7 +1522,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
 
                 // 3. If fetchParams is canceled, then return the appropriate network error for fetchParams.
                 if (fetch_params.is_canceled()) {
-                    returned_pending_response->resolve(Infrastructure::Response::appropriate_network_error(vm, fetch_params));
+                    returned_pending_response->resolve(TRY_OR_IGNORE(Infrastructure::Response::appropriate_network_error(vm, fetch_params)));
                     return;
                 }
 
@@ -1546,7 +1548,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> http_network_or_cache_fet
             ) {
                 // 1. If fetchParams is canceled, then return the appropriate network error for fetchParams.
                 if (fetch_params.is_canceled()) {
-                    returned_pending_response->resolve(Infrastructure::Response::appropriate_network_error(vm, fetch_params));
+                    returned_pending_response->resolve(TRY_OR_IGNORE(Infrastructure::Response::appropriate_network_error(vm, fetch_params)));
                     return;
                 }
                 // 2. Set response to the result of running HTTP-network-or-cache fetch given fetchParams,
@@ -1648,13 +1650,13 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> nonstandard_resource_load
             dbgln_if(WEB_FETCH_DEBUG, "Fetch: ResourceLoader load for '{}' complete", request->url());
             if constexpr (WEB_FETCH_DEBUG)
                 log_response(status_code, response_headers, data);
-            auto [body, _] = MUST(extract_body(realm, data));
+            auto [body, _] = TRY_OR_IGNORE(extract_body(realm, data));
             auto response = Infrastructure::Response::create(vm);
             response->set_status(status_code.value_or(200));
             response->set_body(move(body));
             for (auto const& [name, value] : response_headers) {
-                auto header = Infrastructure::Header::from_string_pair(name, value).release_value_but_fixme_should_propagate_errors();
-                response->header_list()->append(header).release_value_but_fixme_should_propagate_errors();
+                auto header = TRY_OR_IGNORE(Infrastructure::Header::from_string_pair(name, value));
+                TRY_OR_IGNORE(response->header_list()->append(header));
             }
             // FIXME: Set response status message
             pending_response->resolve(response);
@@ -1664,7 +1666,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> nonstandard_resource_load
             auto response = Infrastructure::Response::create(vm);
             // FIXME: This is ugly, ResourceLoader should tell us.
             if (status_code.value_or(0) == 0) {
-                response = Infrastructure::Response::network_error(vm, "HTTP request failed"sv);
+                response = Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("HTTP request failed"_string));
             } else {
                 response->set_status(status_code.value_or(200));
                 // FIXME: Set response status message and body
@@ -1754,12 +1756,12 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> cors_preflight_fetch(JS::
 
             // 3. If either methods or headerNames is failure, return a network error.
             if (methods_or_failure.has<Infrastructure::ExtractHeaderParseFailure>()) {
-                returned_pending_response->resolve(Infrastructure::Response::network_error(vm, "The Access-Control-Allow-Methods in the CORS-preflight response is syntactically invalid"sv));
+                returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("The Access-Control-Allow-Methods in the CORS-preflight response is syntactically invalid"_string)));
                 return;
             }
 
             if (header_names_or_failure.has<Infrastructure::ExtractHeaderParseFailure>()) {
-                returned_pending_response->resolve(Infrastructure::Response::network_error(vm, "The Access-Control-Allow-Headers in the CORS-preflight response is syntactically invalid"sv));
+                returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("The Access-Control-Allow-Headers in the CORS-preflight response is syntactically invalid"_string)));
                 return;
             }
 
@@ -1810,12 +1812,12 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> cors_preflight_fetch(JS::
             //    is "include" or methods does not contain `*`, then return a network error.
             if (!methods.contains_slow(request.method()) && !Infrastructure::is_cors_safelisted_method(request.method())) {
                 if (request.credentials_mode() == Infrastructure::Request::CredentialsMode::Include) {
-                    returned_pending_response->resolve(Infrastructure::Response::network_error(vm, DeprecatedString::formatted("Non-CORS-safelisted method '{}' not found in the CORS-preflight response's Access-Control-Allow-Methods header (the header may be missing). '*' is not allowed as the main request includes credentials."sv, StringView { request.method() })));
+                    returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE(String::formatted("Non-CORS-safelisted method '{}' not found in the CORS-preflight response's Access-Control-Allow-Methods header (the header may be missing). '*' is not allowed as the main request includes credentials."sv, StringView { request.method() }))));
                     return;
                 }
 
                 if (!methods.contains_slow("*"sv.bytes())) {
-                    returned_pending_response->resolve(Infrastructure::Response::network_error(vm, DeprecatedString::formatted("Non-CORS-safelisted method '{}' not found in the CORS-preflight response's Access-Control-Allow-Methods header and there was no '*' entry. The header may be missing."sv, StringView { request.method() })));
+                    returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE(String::formatted("Non-CORS-safelisted method '{}' not found in the CORS-preflight response's Access-Control-Allow-Methods header and there was no '*' entry. The header may be missing."sv, StringView { request.method() }))));
                     return;
                 }
             }
@@ -1834,7 +1836,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> cors_preflight_fetch(JS::
                     }
 
                     if (!is_in_header_names) {
-                        returned_pending_response->resolve(Infrastructure::Response::network_error(vm, DeprecatedString::formatted("Main request contains the header '{}' that is not specified in the CORS-preflight response's Access-Control-Allow-Headers header (the header may be missing). '*' does not capture this header."sv, StringView { header.name })));
+                        returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE(String::formatted("Main request contains the header '{}' that is not specified in the CORS-preflight response's Access-Control-Allow-Headers header (the header may be missing). '*' does not capture this header."sv, StringView { header.name }))));
                         return;
                     }
                 }
@@ -1856,12 +1858,12 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> cors_preflight_fetch(JS::
 
                 if (!is_in_header_names) {
                     if (request.credentials_mode() == Infrastructure::Request::CredentialsMode::Include) {
-                        returned_pending_response->resolve(Infrastructure::Response::network_error(vm, DeprecatedString::formatted("CORS-unsafe request-header '{}' not found in the CORS-preflight response's Access-Control-Allow-Headers header (the header may be missing). '*' is not allowed as the main request includes credentials."sv, StringView { unsafe_name })));
+                        returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE(String::formatted("CORS-unsafe request-header '{}' not found in the CORS-preflight response's Access-Control-Allow-Headers header (the header may be missing). '*' is not allowed as the main request includes credentials."sv, StringView { unsafe_name }))));
                         return;
                     }
 
                     if (!header_names.contains_slow("*"sv.bytes())) {
-                        returned_pending_response->resolve(Infrastructure::Response::network_error(vm, DeprecatedString::formatted("CORS-unsafe request-header '{}' not found in the CORS-preflight response's Access-Control-Allow-Headers header and there was no '*' entry. The header may be missing."sv, StringView { unsafe_name })));
+                        returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE(String::formatted("CORS-unsafe request-header '{}' not found in the CORS-preflight response's Access-Control-Allow-Headers header and there was no '*' entry. The header may be missing."sv, StringView { unsafe_name }))));
                         return;
                     }
                 }
@@ -1888,7 +1890,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<PendingResponse>> cors_preflight_fetch(JS::
         }
 
         // 8. Otherwise, return a network error.
-        returned_pending_response->resolve(Infrastructure::Response::network_error(vm, "CORS-preflight check failed"));
+        returned_pending_response->resolve(Infrastructure::Response::network_error(vm, TRY_OR_IGNORE("CORS-preflight check failed"_string)));
     });
 
     return returned_pending_response;
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/FetchTimingInfo.h b/Userland/Libraries/LibWeb/Fetch/Infrastructure/FetchTimingInfo.h
index ad9698c237..a1d6fff66c 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/FetchTimingInfo.h
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/FetchTimingInfo.h
@@ -1,12 +1,12 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
-#include <AK/DeprecatedString.h>
+#include <AK/String.h>
 #include <AK/Vector.h>
 #include <LibJS/Forward.h>
 #include <LibJS/Heap/Cell.h>
@@ -50,9 +50,9 @@ public:
     [[nodiscard]] JS::GCPtr<ConnectionTimingInfo> final_connection_timing_info() const { return m_final_connection_timing_info; }
     void set_final_connection_timing_info(JS::GCPtr<ConnectionTimingInfo> final_connection_timing_info) { m_final_connection_timing_info = final_connection_timing_info; }
 
-    [[nodiscard]] Vector<DeprecatedString>& server_timing_headers() { return m_server_timing_headers; }
-    [[nodiscard]] Vector<DeprecatedString> const& server_timing_headers() const { return m_server_timing_headers; }
-    void set_server_timing_headers(Vector<DeprecatedString> server_timing_headers) { m_server_timing_headers = move(server_timing_headers); }
+    [[nodiscard]] Vector<String>& server_timing_headers() { return m_server_timing_headers; }
+    [[nodiscard]] Vector<String> const& server_timing_headers() const { return m_server_timing_headers; }
+    void set_server_timing_headers(Vector<String> server_timing_headers) { m_server_timing_headers = move(server_timing_headers); }
 
     [[nodiscard]] bool render_blocking() const { return m_render_blocking; }
     void set_render_blocking(bool render_blocking) { m_render_blocking = render_blocking; }
@@ -110,7 +110,7 @@ private:
     // https://fetch.spec.whatwg.org/#fetch-timing-info-server-timing-headers
     // server-timing headers (default « »)
     //     A list of strings.
-    Vector<DeprecatedString> m_server_timing_headers;
+    Vector<String> m_server_timing_headers;
 
     // https://fetch.spec.whatwg.org/#fetch-timing-info-render-blocking
     // render-blocking (default false)
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP.cpp b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP.cpp
index 1f7338bd0a..8f8df4c3a8 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP.cpp
@@ -4,15 +4,15 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include <AK/DeprecatedString.h>
 #include <AK/GenericLexer.h>
+#include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <LibWeb/Fetch/Infrastructure/HTTP.h>
 
 namespace Web::Fetch::Infrastructure {
 
 // https://fetch.spec.whatwg.org/#collect-an-http-quoted-string
-DeprecatedString collect_an_http_quoted_string(GenericLexer& lexer, HttpQuotedStringExtractValue extract_value)
+ErrorOr<String> collect_an_http_quoted_string(GenericLexer& lexer, HttpQuotedStringExtractValue extract_value)
 {
     // To collect an HTTP quoted string from a string input, given a position variable position and optionally an extract-value flag, run these steps:
     // 1. Let positionStart be position.
@@ -69,13 +69,13 @@ DeprecatedString collect_an_http_quoted_string(GenericLexer& lexer, HttpQuotedSt
 
     // 6. If the extract-value flag is set, then return value.
     if (extract_value == HttpQuotedStringExtractValue::Yes)
-        return value.to_deprecated_string();
+        return value.to_string();
 
     // 7. Return the code points from positionStart to position, inclusive, within input.
     auto position = lexer.tell();
     auto number_of_characters_to_consume = position - position_start + 1;
     lexer.retreat(number_of_characters_to_consume);
-    return lexer.consume(number_of_characters_to_consume);
+    return String::from_utf8(lexer.consume(number_of_characters_to_consume));
 }
 
 }
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP.h b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP.h
index c0d47108c0..eee29c4faf 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP.h
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  * Copyright (c) 2022, Luke Wilde <lukew@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
@@ -37,6 +37,6 @@ enum class HttpQuotedStringExtractValue {
     Yes,
 };
 
-[[nodiscard]] DeprecatedString collect_an_http_quoted_string(GenericLexer& lexer, HttpQuotedStringExtractValue extract_value = HttpQuotedStringExtractValue::No);
+ErrorOr<String> collect_an_http_quoted_string(GenericLexer& lexer, HttpQuotedStringExtractValue extract_value = HttpQuotedStringExtractValue::No);
 
 }
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Bodies.cpp b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Bodies.cpp
index dee3e3f54d..019f3be8ca 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Bodies.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Bodies.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -41,14 +41,15 @@ WebIDL::ExceptionOr<Body> Body::clone() const
 }
 
 // https://fetch.spec.whatwg.org/#fully-reading-body-as-promise
-JS::NonnullGCPtr<WebIDL::Promise> Body::fully_read_as_promise() const
+WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> Body::fully_read_as_promise() const
 {
     auto& vm = Bindings::main_thread_vm();
     auto& realm = *vm.current_realm();
 
     // FIXME: Implement the streams spec - this is completely made up for now :^)
     if (auto const* byte_buffer = m_source.get_pointer<ByteBuffer>()) {
-        auto result = DeprecatedString::copy(*byte_buffer);
+        // FIXME: The buffer may or may not be valid UTF-8.
+        auto result = TRY_OR_THROW_OOM(vm, String::from_utf8(*byte_buffer));
         return WebIDL::create_resolved_promise(realm, JS::PrimitiveString::create(vm, move(result)));
     }
     // Empty, Blob, FormData
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Bodies.h b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Bodies.h
index a8787f0aa7..616993ba5b 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Bodies.h
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Bodies.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -31,9 +31,9 @@ public:
     [[nodiscard]] SourceType const& source() const { return m_source; }
     [[nodiscard]] Optional<u64> const& length() const { return m_length; }
 
-    [[nodiscard]] WebIDL::ExceptionOr<Body> clone() const;
+    WebIDL::ExceptionOr<Body> clone() const;
 
-    [[nodiscard]] JS::NonnullGCPtr<WebIDL::Promise> fully_read_as_promise() const;
+    WebIDL::ExceptionOr<JS::NonnullGCPtr<WebIDL::Promise>> fully_read_as_promise() const;
 
 private:
     // https://fetch.spec.whatwg.org/#concept-body-stream
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp
index abc8d56889..d80844d520 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp
@@ -87,7 +87,7 @@ ErrorOr<Optional<ByteBuffer>> HeaderList::get(ReadonlyBytes name) const
 }
 
 // https://fetch.spec.whatwg.org/#concept-header-list-get-decode-split
-ErrorOr<Optional<Vector<DeprecatedString>>> HeaderList::get_decode_and_split(ReadonlyBytes name) const
+ErrorOr<Optional<Vector<String>>> HeaderList::get_decode_and_split(ReadonlyBytes name) const
 {
     // To get, decode, and split a header name name from header list list, run these steps:
 
@@ -96,14 +96,14 @@ ErrorOr<Optional<Vector<DeprecatedString>>> HeaderList::get_decode_and_split(Rea
 
     // 2. If value is null, then return null.
     if (!value.has_value())
-        return Optional<Vector<DeprecatedString>> {};
+        return Optional<Vector<String>> {};
 
     // 3. Return the result of getting, decoding, and splitting value.
     return get_decode_and_split_header_value(*value);
 }
 
 // https://fetch.spec.whatwg.org/#header-value-get-decode-and-split
-ErrorOr<Optional<Vector<DeprecatedString>>> get_decode_and_split_header_value(ReadonlyBytes value)
+ErrorOr<Optional<Vector<String>>> get_decode_and_split_header_value(ReadonlyBytes value)
 {
     // To get, decode, and split a header value value, run these steps:
 
@@ -114,7 +114,7 @@ ErrorOr<Optional<Vector<DeprecatedString>>> get_decode_and_split_header_value(Re
     auto lexer = GenericLexer { input };
 
     // 3. Let values be a list of strings, initially empty.
-    Vector<DeprecatedString> values;
+    Vector<String> values;
 
     // 4. Let temporaryValue be the empty string.
     StringBuilder temporary_value_builder;
@@ -123,14 +123,14 @@ ErrorOr<Optional<Vector<DeprecatedString>>> get_decode_and_split_header_value(Re
     while (!lexer.is_eof()) {
         // 1. Append the result of collecting a sequence of code points that are not U+0022 (") or U+002C (,) from input, given position, to temporaryValue.
         // NOTE: The result might be the empty string.
-        temporary_value_builder.append(lexer.consume_until(is_any_of("\","sv)));
+        TRY(temporary_value_builder.try_append(lexer.consume_until(is_any_of("\","sv))));
 
         // 2. If position is not past the end of input, then:
         if (!lexer.is_eof()) {
             // 1. If the code point at position within input is U+0022 ("), then:
             if (lexer.peek() == '"') {
                 // 1. Append the result of collecting an HTTP quoted string from input, given position, to temporaryValue.
-                temporary_value_builder.append(collect_an_http_quoted_string(lexer));
+                TRY(temporary_value_builder.try_append(TRY(collect_an_http_quoted_string(lexer))));
 
                 // 2. If position is not past the end of input, then continue.
                 if (!lexer.is_eof())
@@ -147,10 +147,10 @@ ErrorOr<Optional<Vector<DeprecatedString>>> get_decode_and_split_header_value(Re
         }
 
         // 3. Remove all HTTP tab or space from the start and end of temporaryValue.
-        auto temporary_value = temporary_value_builder.to_deprecated_string().trim(HTTP_TAB_OR_SPACE, TrimMode::Both);
+        auto temporary_value = TRY(String::from_utf8(temporary_value_builder.string_view().trim(HTTP_TAB_OR_SPACE, TrimMode::Both)));
 
         // 4. Append temporaryValue to values.
-        values.append(move(temporary_value));
+        TRY(values.try_append(move(temporary_value)));
 
         // 5. Set temporaryValue to the empty string.
         temporary_value_builder.clear();
@@ -301,10 +301,10 @@ ErrorOr<Vector<Header>> HeaderList::sort_and_combine() const
 Optional<MimeSniff::MimeType> HeaderList::extract_mime_type() const
 {
     // 1. Let charset be null.
-    Optional<DeprecatedString> charset;
+    Optional<String> charset;
 
     // 2. Let essence be null.
-    Optional<DeprecatedString> essence;
+    Optional<String> essence;
 
     // 3. Let mimeType be null.
     Optional<MimeSniff::MimeType> mime_type;
@@ -332,21 +332,21 @@ Optional<MimeSniff::MimeType> HeaderList::extract_mime_type() const
         mime_type = temporary_mime_type;
 
         // 4. If mimeType’s essence is not essence, then:
-        if (mime_type->essence() != essence) {
+        if (!essence.has_value() || (mime_type->essence() != essence->bytes_as_string_view())) {
             // 1. Set charset to null.
             charset = {};
 
             // 2. If mimeType’s parameters["charset"] exists, then set charset to mimeType’s parameters["charset"].
-            auto charset_it = mime_type->parameters().find("charset"sv);
-            if (charset_it != mime_type->parameters().end())
-                charset = charset_it->value;
+            auto it = mime_type->parameters().find("charset"sv);
+            if (it != mime_type->parameters().end())
+                charset = String::from_deprecated_string(it->value).release_value_but_fixme_should_propagate_errors();
 
             // 3. Set essence to mimeType’s essence.
-            essence = mime_type->essence();
+            essence = String::from_deprecated_string(mime_type->essence()).release_value_but_fixme_should_propagate_errors();
         }
         // 5. Otherwise, if mimeType’s parameters["charset"] does not exist, and charset is non-null, set mimeType’s parameters["charset"] to charset.
         else if (!mime_type->parameters().contains("charset"sv) && charset.has_value()) {
-            mime_type->set_parameter("charset"sv, charset.value());
+            mime_type->set_parameter("charset"sv, charset->to_deprecated_string());
         }
     }
 
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.h b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.h
index 7e5b31b7f7..1bfd06d9b5 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.h
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -7,11 +7,11 @@
 #pragma once
 
 #include <AK/ByteBuffer.h>
-#include <AK/DeprecatedString.h>
 #include <AK/Error.h>
 #include <AK/Forward.h>
 #include <AK/HashTable.h>
 #include <AK/Optional.h>
+#include <AK/String.h>
 #include <AK/Vector.h>
 #include <LibJS/Forward.h>
 #include <LibJS/Heap/Cell.h>
@@ -45,7 +45,7 @@ public:
 
     [[nodiscard]] bool contains(ReadonlyBytes) const;
     [[nodiscard]] ErrorOr<Optional<ByteBuffer>> get(ReadonlyBytes) const;
-    [[nodiscard]] ErrorOr<Optional<Vector<DeprecatedString>>> get_decode_and_split(ReadonlyBytes) const;
+    [[nodiscard]] ErrorOr<Optional<Vector<String>>> get_decode_and_split(ReadonlyBytes) const;
     [[nodiscard]] ErrorOr<void> append(Header);
     void delete_(ReadonlyBytes name);
     [[nodiscard]] ErrorOr<void> set(Header);
@@ -62,7 +62,7 @@ struct RangeHeaderValue {
 struct ExtractHeaderParseFailure {
 };
 
-[[nodiscard]] ErrorOr<Optional<Vector<DeprecatedString>>> get_decode_and_split_header_value(ReadonlyBytes);
+[[nodiscard]] ErrorOr<Optional<Vector<String>>> get_decode_and_split_header_value(ReadonlyBytes);
 [[nodiscard]] ErrorOr<OrderedHashTable<ByteBuffer>> convert_header_names_to_a_sorted_lowercase_set(Span<ReadonlyBytes>);
 [[nodiscard]] bool is_header_name(ReadonlyBytes);
 [[nodiscard]] bool is_header_value(ReadonlyBytes);
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.cpp b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.cpp
index a8bdd656a1..2dd6983a7e 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -177,21 +177,21 @@ bool Request::has_redirect_tainted_origin() const
 }
 
 // https://fetch.spec.whatwg.org/#serializing-a-request-origin
-DeprecatedString Request::serialize_origin() const
+ErrorOr<String> Request::serialize_origin() const
 {
     // 1. If request has a redirect-tainted origin, then return "null".
     if (has_redirect_tainted_origin())
-        return "null"sv;
+        return "null"_string;
 
     // 2. Return request’s origin, serialized.
-    return m_origin.get<HTML::Origin>().serialize();
+    return String::from_deprecated_string(m_origin.get<HTML::Origin>().serialize());
 }
 
 // https://fetch.spec.whatwg.org/#byte-serializing-a-request-origin
 ErrorOr<ByteBuffer> Request::byte_serialize_origin() const
 {
     // Byte-serializing a request origin, given a request request, is to return the result of serializing a request origin with request, isomorphic encoded.
-    return ByteBuffer::copy(serialize_origin().bytes());
+    return ByteBuffer::copy(TRY(serialize_origin()).bytes());
 }
 
 // https://fetch.spec.whatwg.org/#concept-request-clone
@@ -260,14 +260,14 @@ ErrorOr<void> Request::add_range_header(u64 first, Optional<u64> const& last)
     auto range_value = MUST(ByteBuffer::copy("bytes"sv.bytes()));
 
     // 3. Serialize and isomorphic encode first, and append the result to rangeValue.
-    TRY(range_value.try_append(DeprecatedString::number(first).bytes()));
+    TRY(range_value.try_append(TRY(String::number(first)).bytes()));
 
     // 4. Append 0x2D (-) to rangeValue.
     TRY(range_value.try_append('-'));
 
     // 5. If last is given, then serialize and isomorphic encode it, and append the result to rangeValue.
     if (last.has_value())
-        TRY(range_value.try_append(DeprecatedString::number(*last).bytes()));
+        TRY(range_value.try_append(TRY(String::number(*last)).bytes()));
 
     // 6. Append (`Range`, rangeValue) to request’s header list.
     auto header = Header {
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.h b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.h
index 495cf2f3f7..22db8d7fe3 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.h
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -7,10 +7,10 @@
 #pragma once
 
 #include <AK/ByteBuffer.h>
-#include <AK/DeprecatedString.h>
 #include <AK/Error.h>
 #include <AK/Forward.h>
 #include <AK/Optional.h>
+#include <AK/String.h>
 #include <AK/URL.h>
 #include <AK/Variant.h>
 #include <AK/Vector.h>
@@ -186,8 +186,8 @@ public:
     [[nodiscard]] ReservedClientType& reserved_client() { return m_reserved_client; }
     void set_reserved_client(ReservedClientType reserved_client) { m_reserved_client = move(reserved_client); }
 
-    [[nodiscard]] DeprecatedString const& replaces_client_id() const { return m_replaces_client_id; }
-    void set_replaces_client_id(DeprecatedString replaces_client_id) { m_replaces_client_id = move(replaces_client_id); }
+    [[nodiscard]] String const& replaces_client_id() const { return m_replaces_client_id; }
+    void set_replaces_client_id(String replaces_client_id) { m_replaces_client_id = move(replaces_client_id); }
 
     [[nodiscard]] WindowType const& window() const { return m_window; }
     void set_window(WindowType window) { m_window = move(window); }
@@ -234,11 +234,11 @@ public:
     [[nodiscard]] RedirectMode redirect_mode() const { return m_redirect_mode; }
     void set_redirect_mode(RedirectMode redirect_mode) { m_redirect_mode = redirect_mode; }
 
-    [[nodiscard]] DeprecatedString const& integrity_metadata() const { return m_integrity_metadata; }
-    void set_integrity_metadata(DeprecatedString integrity_metadata) { m_integrity_metadata = move(integrity_metadata); }
+    [[nodiscard]] String const& integrity_metadata() const { return m_integrity_metadata; }
+    void set_integrity_metadata(String integrity_metadata) { m_integrity_metadata = move(integrity_metadata); }
 
-    [[nodiscard]] DeprecatedString const& cryptographic_nonce_metadata() const { return m_cryptographic_nonce_metadata; }
-    void set_cryptographic_nonce_metadata(DeprecatedString cryptographic_nonce_metadata) { m_cryptographic_nonce_metadata = move(cryptographic_nonce_metadata); }
+    [[nodiscard]] String const& cryptographic_nonce_metadata() const { return m_cryptographic_nonce_metadata; }
+    void set_cryptographic_nonce_metadata(String cryptographic_nonce_metadata) { m_cryptographic_nonce_metadata = move(cryptographic_nonce_metadata); }
 
     [[nodiscard]] Optional<ParserMetadata> const& parser_metadata() const { return m_parser_metadata; }
     void set_parser_metadata(Optional<ParserMetadata> parser_metadata) { m_parser_metadata = move(parser_metadata); }
@@ -294,7 +294,7 @@ public:
 
     [[nodiscard]] bool has_redirect_tainted_origin() const;
 
-    [[nodiscard]] DeprecatedString serialize_origin() const;
+    [[nodiscard]] ErrorOr<String> serialize_origin() const;
     [[nodiscard]] ErrorOr<ByteBuffer> byte_serialize_origin() const;
 
     [[nodiscard]] WebIDL::ExceptionOr<JS::NonnullGCPtr<Request>> clone(JS::VM&) const;
@@ -352,7 +352,7 @@ private:
 
     // https://fetch.spec.whatwg.org/#concept-request-replaces-client-id
     // A request has an associated replaces client id (a string). Unless stated otherwise it is the empty string.
-    DeprecatedString m_replaces_client_id { DeprecatedString::empty() };
+    String m_replaces_client_id;
 
     // https://fetch.spec.whatwg.org/#concept-request-window
     // A request has an associated window ("no-window", "client", or an environment settings object whose global object
@@ -444,12 +444,12 @@ private:
 
     // https://fetch.spec.whatwg.org/#concept-request-integrity-metadata
     // A request has associated integrity metadata (a string). Unless stated otherwise, it is the empty string.
-    DeprecatedString m_integrity_metadata { DeprecatedString::empty() };
+    String m_integrity_metadata;
 
     // https://fetch.spec.whatwg.org/#concept-request-nonce-metadata
     // A request has associated cryptographic nonce metadata (a string). Unless stated otherwise, it is the empty
     // string.
-    DeprecatedString m_cryptographic_nonce_metadata { DeprecatedString::empty() };
+    String m_cryptographic_nonce_metadata;
 
     // https://fetch.spec.whatwg.org/#concept-request-parser-metadata
     // A request has associated parser metadata which is the empty string, "parser-inserted", or
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Responses.cpp b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Responses.cpp
index 237bdd1b63..a0bec94dbf 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Responses.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Responses.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -37,14 +37,14 @@ JS::NonnullGCPtr<Response> Response::create(JS::VM& vm)
 // A network error is a response whose status is always 0, status message is always
 // the empty byte sequence, header list is always empty, and body is always null.
 
-JS::NonnullGCPtr<Response> Response::aborted_network_error(JS::VM& vm)
+ErrorOr<JS::NonnullGCPtr<Response>> Response::aborted_network_error(JS::VM& vm)
 {
-    auto response = network_error(vm, "Fetch has been aborted"sv);
+    auto response = network_error(vm, TRY("Fetch has been aborted"_string));
     response->set_aborted(true);
     return response;
 }
 
-JS::NonnullGCPtr<Response> Response::network_error(JS::VM& vm, DeprecatedString message)
+JS::NonnullGCPtr<Response> Response::network_error(JS::VM& vm, String message)
 {
     dbgln_if(WEB_FETCH_DEBUG, "Fetch: Creating network error response with message: {}", message);
     auto response = Response::create(vm);
@@ -56,15 +56,15 @@ JS::NonnullGCPtr<Response> Response::network_error(JS::VM& vm, DeprecatedString
 }
 
 // https://fetch.spec.whatwg.org/#appropriate-network-error
-JS::NonnullGCPtr<Response> Response::appropriate_network_error(JS::VM& vm, FetchParams const& fetch_params)
+ErrorOr<JS::NonnullGCPtr<Response>> Response::appropriate_network_error(JS::VM& vm, FetchParams const& fetch_params)
 {
     // 1. Assert: fetchParams is canceled.
     VERIFY(fetch_params.is_canceled());
 
     // 2. Return an aborted network error if fetchParams is aborted; otherwise return a network error.
     return fetch_params.is_aborted()
-        ? aborted_network_error(vm)
-        : network_error(vm, "Fetch has been terminated"sv);
+        ? TRY(aborted_network_error(vm))
+        : network_error(vm, TRY("Fetch has been terminated"_string));
 }
 
 // https://fetch.spec.whatwg.org/#concept-aborted-network-error
@@ -94,7 +94,7 @@ Optional<AK::URL const&> Response::url() const
 }
 
 // https://fetch.spec.whatwg.org/#concept-response-location-url
-ErrorOr<Optional<AK::URL>> Response::location_url(Optional<DeprecatedString> const& request_fragment) const
+ErrorOr<Optional<AK::URL>> Response::location_url(Optional<String> const& request_fragment) const
 {
     // The location URL of a response response, given null or an ASCII string requestFragment, is the value returned by the following steps. They return null, failure, or a URL.
 
@@ -120,7 +120,7 @@ ErrorOr<Optional<AK::URL>> Response::location_url(Optional<DeprecatedString> con
 
     // 4. If location is a URL whose fragment is null, then set location’s fragment to requestFragment.
     if (location.fragment().is_null())
-        location.set_fragment(request_fragment.value_or({}));
+        location.set_fragment(request_fragment.has_value() ? request_fragment->to_deprecated_string() : DeprecatedString {});
 
     // 5. Return location.
     return location;
diff --git a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Responses.h b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Responses.h
index 63a6197e8f..4ed881d88f 100644
--- a/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Responses.h
+++ b/Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Responses.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2022-2023, Linus Groh <linusg@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -50,9 +50,9 @@ public:
     };
 
     [[nodiscard]] static JS::NonnullGCPtr<Response> create(JS::VM&);
-    [[nodiscard]] static JS::NonnullGCPtr<Response> aborted_network_error(JS::VM&);
-    [[nodiscard]] static JS::NonnullGCPtr<Response> network_error(JS::VM&, DeprecatedString message);
-    [[nodiscard]] static JS::NonnullGCPtr<Response> appropriate_network_error(JS::VM&, FetchParams const&);
+    static ErrorOr<JS::NonnullGCPtr<Response>> aborted_network_error(JS::VM&);
+    [[nodiscard]] static JS::NonnullGCPtr<Response> network_error(JS::VM&, String message);
+    static ErrorOr<JS::NonnullGCPtr<Response>> appropriate_network_error(JS::VM&, FetchParams const&);
 
     virtual ~Response() = default;
 
@@ -104,12 +104,12 @@ public:
     [[nodiscard]] bool is_network_error() const;
 
     [[nodiscard]] Optional<AK::URL const&> url() const;
-    [[nodiscard]] ErrorOr<Optional<AK::URL>> location_url(Optional<DeprecatedString> const& request_fragment) const;
+    [[nodiscard]] ErrorOr<Optional<AK::URL>> location_url(Optional<String> const& request_fragment) const;
 
     [[nodiscard]] WebIDL::ExceptionOr<JS::NonnullGCPtr<Response>> clone(JS::VM&) const;
 
     // Non-standard
-    Optional<DeprecatedString> const& network_error_message() const { return m_network_error_message; }
+    Optional<String> const& network_error_message() const { return m_network_error_message; }
 
 protected:
     explicit Response(JS::NonnullGCPtr<HeaderList>);
@@ -177,7 +177,7 @@ private:
     bool m_has_cross_origin_redirects { false };
 
     // Non-standard
-    Optional<DeprecatedString> m_network_error_message;
+    Optional<String> m_network_error_message;
 };
 
 // https://fetch.spec.whatwg.org/#concept-filtered-response
diff --git a/Userland/Libraries/LibWeb/Fetch/Request.cpp b/Userland/Libraries/LibWeb/Fetch/Request.cpp
index 81d9d218a9..dccc365a59 100644
--- a/Userland/Libraries/LibWeb/Fetch/Request.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Request.cpp
@@ -355,7 +355,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<Request>> Request::construct_impl(JS::Realm
 
     // 23. If init["integrity"] exists, then set request’s integrity metadata to it.
     if (init.integrity.has_value())
-        request->set_integrity_metadata(init.integrity->to_deprecated_string());
+        request->set_integrity_metadata(*init.integrity);
 
     // 24. If init["keepalive"] exists, then set request’s keepalive to it.
     if (init.keepalive.has_value())
@@ -590,12 +590,10 @@ Bindings::RequestRedirect Request::redirect() const
 }
 
 // https://fetch.spec.whatwg.org/#dom-request-integrity
-WebIDL::ExceptionOr<String> Request::integrity() const
+String Request::integrity() const
 {
-    auto& vm = this->vm();
-
     // The integrity getter steps are to return this’s request’s integrity metadata.
-    return TRY_OR_THROW_OOM(vm, String::from_deprecated_string(m_request->integrity_metadata()));
+    return m_request->integrity_metadata();
 }
 
 // https://fetch.spec.whatwg.org/#dom-request-keepalive
diff --git a/Userland/Libraries/LibWeb/Fetch/Request.h b/Userland/Libraries/LibWeb/Fetch/Request.h
index ee9d0709e0..c15f226b27 100644
--- a/Userland/Libraries/LibWeb/Fetch/Request.h
+++ b/Userland/Libraries/LibWeb/Fetch/Request.h
@@ -88,7 +88,7 @@ public:
     [[nodiscard]] Bindings::RequestCredentials credentials() const;
     [[nodiscard]] Bindings::RequestCache cache() const;
     [[nodiscard]] Bindings::RequestRedirect redirect() const;
-    [[nodiscard]] WebIDL::ExceptionOr<String> integrity() const;
+    [[nodiscard]] String integrity() const;
     [[nodiscard]] bool keepalive() const;
     [[nodiscard]] bool is_reload_navigation() const;
     [[nodiscard]] bool is_history_navigation() const;
diff --git a/Userland/Libraries/LibWeb/Fetch/Response.cpp b/Userland/Libraries/LibWeb/Fetch/Response.cpp
index 54e7cf0e79..08ba1d58d0 100644
--- a/Userland/Libraries/LibWeb/Fetch/Response.cpp
+++ b/Userland/Libraries/LibWeb/Fetch/Response.cpp
@@ -160,11 +160,11 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<Response>> Response::construct_impl(JS::Rea
 }
 
 // https://fetch.spec.whatwg.org/#dom-response-error
-JS::NonnullGCPtr<Response> Response::error(JS::VM& vm)
+WebIDL::ExceptionOr<JS::NonnullGCPtr<Response>> Response::error(JS::VM& vm)
 {
     // The static error() method steps are to return the result of creating a Response object, given a new network error, "immutable", and this’s relevant Realm.
     // FIXME: How can we reliably get 'this', i.e. the object the function was called on, in IDL-defined functions?
-    return Response::create(*vm.current_realm(), Infrastructure::Response::network_error(vm, "Response created via `Response.error()`"sv), Headers::Guard::Immutable).release_value_but_fixme_should_propagate_errors();
+    return Response::create(*vm.current_realm(), Infrastructure::Response::network_error(vm, TRY_OR_THROW_OOM(vm, "Response created via `Response.error()`"_string)), Headers::Guard::Immutable);
 }
 
 // https://fetch.spec.whatwg.org/#dom-response-redirect
diff --git a/Userland/Libraries/LibWeb/Fetch/Response.h b/Userland/Libraries/LibWeb/Fetch/Response.h
index 4ae12d7624..1f6152fc86 100644
--- a/Userland/Libraries/LibWeb/Fetch/Response.h
+++ b/Userland/Libraries/LibWeb/Fetch/Response.h
@@ -45,7 +45,7 @@ public:
     [[nodiscard]] JS::NonnullGCPtr<Infrastructure::Response> response() const { return m_response; }
 
     // JS API functions
-    [[nodiscard]] static JS::NonnullGCPtr<Response> error(JS::VM&);
+    static WebIDL::ExceptionOr<JS::NonnullGCPtr<Response>> error(JS::VM&);
     static WebIDL::ExceptionOr<JS::NonnullGCPtr<Response>> redirect(JS::VM&, String const& url, u16 status);
     static WebIDL::ExceptionOr<JS::NonnullGCPtr<Response>> json(JS::VM&, JS::Value data, ResponseInit const& init = {});
     [[nodiscard]] Bindings::ResponseType type() const;