LibWeb: Correct handling of negative step values in nth-foo() selectors

This should be 1% on Acid3. :^)

Added the `-5n+3` case to all `nth-of-whatever()` selector test pages,
so we can easily check that it works.

5 files changed, 123 insertions, 18 deletions

diff --git a/Base/res/html/misc/nth-child.html b/Base/res/html/misc/nth-child.html
index c5e6492dcb..ed1051700b 100644
--- a/Base/res/html/misc/nth-child.html
+++ b/Base/res/html/misc/nth-child.html
@@ -17,7 +17,8 @@
         .plus-n > div:nth-child(+n), /** "+n" is special case inside parser. */
         .minus-n > div:nth-child(-n), /** "-n" is special case inside parser. */
         ._0n-plus-1 > div:nth-child(-0n+1),
-        .n-plus-2__minus-n-plus-4 > div:nth-child(+n + 2 ):nth-child( -n+4) {
+        .n-plus-2__minus-n-plus-4 > div:nth-child(+n + 2 ):nth-child( -n+4),
+        .acid3 > div:nth-child(-5n+3) {
             background-color: lightblue;
         }
     </style>
@@ -136,5 +137,24 @@
     <div>4 +</div>
     <div>5</div>
 </div>
+
+<h4>:nth-child(-5n+3) - Acid3</h4>
+<div class="acid3">
+    <div>1</div>
+    <div>2</div>
+    <div>3+</div>
+    <div>4</div>
+    <div>5</div>
+    <div>6</div>
+    <div>7</div>
+    <div>8</div>
+    <div>9</div>
+    <div>10</div>
+    <div>11</div>
+    <div>12</div>
+    <div>13</div>
+    <div>14</div>
+    <div>15</div>
+</div>
 </body>
 </html>
diff --git a/Base/res/html/misc/nth-last-child.html b/Base/res/html/misc/nth-last-child.html
index 24bff73546..6c514a0a18 100644
--- a/Base/res/html/misc/nth-last-child.html
+++ b/Base/res/html/misc/nth-last-child.html
@@ -17,7 +17,8 @@
         .plus-n > div:nth-last-child(+n), /** "+n" is special case inside parser. */
         .minus-n > div:nth-last-child(-n), /** "-n" is special case inside parser. */
         ._0n-plus-1 > div:nth-last-child(-0n+1),
-        .n-plus-2__minus-n-plus-3 > div:nth-last-child(+n + 2 ):nth-last-child( -n+3) {
+        .n-plus-2__minus-n-plus-3 > div:nth-last-child(+n + 2 ):nth-last-child( -n+3),
+        .acid3 > div:nth-last-child(-5n+3) {
             background-color: lightblue;
         }
     </style>
@@ -136,5 +137,24 @@
     <div>4 +</div>
     <div>5</div>
 </div>
+
+<h4>:nth-last-child(-5n+3) - Acid3</h4>
+<div class="acid3">
+    <div>1</div>
+    <div>2</div>
+    <div>3</div>
+    <div>4</div>
+    <div>5</div>
+    <div>6</div>
+    <div>7</div>
+    <div>8</div>
+    <div>9</div>
+    <div>10</div>
+    <div>11</div>
+    <div>12</div>
+    <div>13+</div>
+    <div>14</div>
+    <div>15</div>
+</div>
 </body>
 </html>
diff --git a/Base/res/html/misc/nth-last-of-type.html b/Base/res/html/misc/nth-last-of-type.html
index 9112a3545a..4b22cc8157 100644
--- a/Base/res/html/misc/nth-last-of-type.html
+++ b/Base/res/html/misc/nth-last-of-type.html
@@ -21,7 +21,8 @@
         .plus-n > p:nth-last-of-type(+n), /** "+n" is special case inside parser. */
         .minus-n > p:nth-last-of-type(-n), /** "-n" is special case inside parser. */
         ._0n-plus-1 > p:nth-last-of-type(-0n+1),
-        .n-plus-2__minus-n-plus-3 > p:nth-last-of-type(+n + 2 ):nth-last-of-type( -n+3) {
+        .n-plus-2__minus-n-plus-3 > p:nth-last-of-type(+n + 2 ):nth-last-of-type( -n+3),
+        .acid3 > p:nth-last-of-type(-5n+3) {
             background-color: lightblue;
         }
     </style>
@@ -171,5 +172,34 @@
     <div>(div)</div>
     <p>5</p>
 </div>
+
+<h4>:nth-last-of-type(-5n+3) - Acid3</h4>
+<div class="acid3">
+    <p>1</p>
+    <div>(div)</div>
+    <div>(div)</div>
+    <p>2</p>
+    <div>(div)</div>
+    <p>3</p>
+    <div>(div)</div>
+    <div>(div)</div>
+    <div>(div)</div>
+    <p>4</p>
+    <p>5</p>
+    <p>6</p>
+    <div>(div)</div>
+    <p>7</p>
+    <div>(div)</div>
+    <div>(div)</div>
+    <p>8</p>
+    <div>(div)</div>
+    <p>9</p>
+    <p>10</p>
+    <p>11</p>
+    <p>12</p>
+    <p>13 +</p>
+    <p>14</p>
+    <p>15</p>
+</div>
 </body>
 </html>
diff --git a/Base/res/html/misc/nth-of-type.html b/Base/res/html/misc/nth-of-type.html
index 1a3bc68f06..d93a525bd4 100644
--- a/Base/res/html/misc/nth-of-type.html
+++ b/Base/res/html/misc/nth-of-type.html
@@ -22,7 +22,8 @@
         .plus-n > p:nth-of-type(+n), /** "+n" is special case inside parser. */
         .minus-n > p:nth-of-type(-n), /** "-n" is special case inside parser. */
         ._0n-plus-1 > p:nth-of-type(-0n+1),
-        .n-plus-2__minus-n-plus-4 > p:nth-of-type(+n + 2 ):nth-of-type( -n+4) {
+        .n-plus-2__minus-n-plus-4 > p:nth-of-type(+n + 2 ):nth-of-type( -n+4),
+        .acid3 > p:nth-of-type(-5n+3) {
             background-color: lightblue;
         }
     </style>
@@ -172,5 +173,34 @@
     <p>4 +</p>
     <p>5</p>
 </div>
+
+<h4>:nth-of-type(-5n+3) - Acid3</h4>
+<div class="acid3">
+    <p>1</p>
+    <div>(div)</div>
+    <div>(div)</div>
+    <p>2</p>
+    <div>(div)</div>
+    <p>3 +</p>
+    <div>(div)</div>
+    <div>(div)</div>
+    <div>(div)</div>
+    <p>4</p>
+    <p>5</p>
+    <p>6</p>
+    <div>(div)</div>
+    <p>7</p>
+    <div>(div)</div>
+    <div>(div)</div>
+    <p>8</p>
+    <div>(div)</div>
+    <p>9</p>
+    <p>10</p>
+    <p>11</p>
+    <p>12</p>
+    <p>13</p>
+    <p>14</p>
+    <p>15</p>
+</div>
 </body>
 </html>
diff --git a/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp b/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
index 52b4965ab9..14f4776149 100644
--- a/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
+++ b/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
@@ -179,13 +179,21 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
             VERIFY_NOT_REACHED();
         }
 
-        if (step_size < 0) {
-            // When "step_size" is negative, selector represents first "offset" elements in document tree.
+        // When "step_size == -1", selector represents first "offset" elements in document tree.
+        if (step_size == -1)
             return !(offset <= 0 || index > offset);
-        } else if (step_size == 1) {
-            // When "step_size == 1", selector represents last "offset" elements in document tree.
+
+        // When "step_size == 1", selector represents last "offset" elements in document tree.
+        if (step_size == 1)
             return !(offset < 0 || index < offset);
-        }
+
+        // When "step_size == 0", selector picks only the "offset" element.
+        if (step_size == 0)
+            return index == offset;
+
+        // If both are negative, nothing can match.
+        if (step_size < 0 && offset < 0)
+            return false;
 
         // Like "a % b", but handles negative integers correctly.
         auto const canonical_modulo = [](int a, int b) -> int {
@@ -196,15 +204,12 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
             return c;
         };
 
-        if (step_size == 0) {
-            // Avoid divide by zero.
-            if (index != offset) {
-                return false;
-            }
-        } else if (canonical_modulo(index - offset, step_size) != 0) {
-            return false;
-        }
-        return true;
+        // When "step_size < 0", we start at "offset" and count backwards.
+        if (step_size < 0)
+            return index <= offset && canonical_modulo(index - offset, -step_size) == 0;
+
+        // Otherwise, we start at "offset" and count forwards.
+        return index >= offset && canonical_modulo(index - offset, step_size) == 0;
     }
 
     return false;