LibWeb: Parse double-position `linear-gradient()` color stops

The only accepted syntax for these seems to be
<color> <length percentage> <length percentage>, no other order.

But that's just gathered from looking at other browsers as though
these are supported by all major browsers, they don't appear in
the W3C spec.

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -2509,15 +2509,16 @@ RefPtr<StyleValue> Parser::parse_linear_gradient_function(ComponentValue const&
         auto& token = tokens.next_token();
 
         Gfx::Color color;
-        Optional<LengthPercentage> length;
+        Optional<LengthPercentage> position;
+        Optional<LengthPercentage> second_position;
         auto dimension = parse_dimension(token);
         if (dimension.has_value() && dimension->is_length_percentage()) {
             // [<length-percentage> <color>] or [<length-percentage>]
-            length = dimension->length_percentage();
+            position = dimension->length_percentage();
             tokens.skip_whitespace();
             // <length-percentage>
             if (!tokens.has_next_token() || tokens.peek_token().is(Token::Type::Comma)) {
-                element.transition_hint = GradientColorHint { *length };
+                element.transition_hint = GradientColorHint { *position };
                 return ElementType::ColorHint;
             }
             // <length-percentage> <color>
@@ -2532,16 +2533,21 @@ RefPtr<StyleValue> Parser::parse_linear_gradient_function(ComponentValue const&
                 return ElementType::Garbage;
             color = *maybe_color;
             tokens.skip_whitespace();
-            if (tokens.has_next_token() && !tokens.peek_token().is(Token::Type::Comma)) {
-                auto token = tokens.next_token();
-                auto dimension = parse_dimension(token);
-                if (!dimension.has_value() || !dimension->is_length_percentage())
-                    return ElementType::Garbage;
-                length = dimension->length_percentage();
+            // Allow up to [<color> <length-percentage> <length-percentage>] (double-position color stops)
+            // Note: Double-position color stops only appear to be valid in this order.
+            for (auto stop_position : Array { &position, &second_position }) {
+                if (tokens.has_next_token() && !tokens.peek_token().is(Token::Type::Comma)) {
+                    auto token = tokens.next_token();
+                    auto dimension = parse_dimension(token);
+                    if (!dimension.has_value() || !dimension->is_length_percentage())
+                        return ElementType::Garbage;
+                    *stop_position = dimension->length_percentage();
+                    tokens.skip_whitespace();
+                }
             }
         }
 
-        element.color_stop = GradientColorStop { color, length };
+        element.color_stop = GradientColorStop { color, position, second_position };
         return ElementType::ColorStop;
     };
 

Userland/Libraries/LibWeb/CSS/StyleValue.cpp

@@ -1512,8 +1512,9 @@ String LinearGradientStyleValue::to_string() const
         }
 
         serialize_a_srgb_value(builder, element.color_stop.color);
-        if (element.color_stop.length.has_value()) {
-            builder.appendff(" {}"sv, element.color_stop.length->to_string());
+        for (auto position : Array { &element.color_stop.position, &element.color_stop.second_position }) {
+            if (position->has_value())
+                builder.appendff(" {}"sv, (*position)->to_string());
         }
         first = false;
     }
@@ -1537,7 +1538,7 @@ static bool operator==(GradientColorHint a, GradientColorHint b)
 
 static bool operator==(GradientColorStop a, GradientColorStop b)
 {
-    return a.color == b.color && a.length == b.length;
+    return a.color == b.color && a.position == b.position && a.second_position == b.second_position;
 }
 
 static bool operator==(ColorStopListElement a, ColorStopListElement b)

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -74,7 +74,8 @@ enum class SideOrCorner {
 
 struct GradientColorStop {
     Color color;
-    Optional<LengthPercentage> length;
+    Optional<LengthPercentage> position;
+    Optional<LengthPercentage> second_position = {};
 };
 
 struct GradientColorHint {

Userland/Libraries/LibWeb/Painting/GradientPainting.cpp

@@ -49,13 +49,13 @@ LinearGradientData resolve_linear_gradient_data(Layout::Node const& node, Gfx::F
 
     // 1. If the first color stop does not have a position, set its position to 0%.
     auto& first_stop = color_stop_list.first().color_stop;
-    resolved_color_stops.first().position = first_stop.length.has_value()
-        ? first_stop.length->resolved(node, gradient_length).to_px(node)
+    resolved_color_stops.first().position = first_stop.position.has_value()
+        ? first_stop.position->resolved(node, gradient_length).to_px(node)
         : 0;
     //    If the last color stop does not have a position, set its position to 100%
     auto& last_stop = color_stop_list.last().color_stop;
-    resolved_color_stops.last().position = last_stop.length.has_value()
-        ? last_stop.length->resolved(node, gradient_length).to_px(node)
+    resolved_color_stops.last().position = last_stop.position.has_value()
+        ? last_stop.position->resolved(node, gradient_length).to_px(node)
         : gradient_length_px;
 
     // 2. If a color stop or transition hint has a position that is less than the
@@ -71,8 +71,8 @@ LinearGradientData resolve_linear_gradient_data(Layout::Node const& node, Gfx::F
             resolved_color_stops[i].transition_hint = value;
             max_previous_color_stop_or_hint = value;
         }
-        if (stop.color_stop.length.has_value()) {
-            float value = stop.color_stop.length->resolved(node, gradient_length).to_px(node);
+        if (stop.color_stop.position.has_value()) {
+            float value = stop.color_stop.position->resolved(node, gradient_length).to_px(node);
             value = max(value, max_previous_color_stop_or_hint);
             resolved_color_stops[i].position = value;
             max_previous_color_stop_or_hint = value;
@@ -86,7 +86,7 @@ LinearGradientData resolve_linear_gradient_data(Layout::Node const& node, Gfx::F
     size_t i = 1;
     auto find_run_end = [&] {
         auto color_stop_has_position = [](auto& color_stop) {
-            return color_stop.transition_hint.has_value() || color_stop.color_stop.length.has_value();
+            return color_stop.transition_hint.has_value() || color_stop.color_stop.position.has_value();
         };
         while (i < color_stop_list.size() - 1 && !color_stop_has_position(color_stop_list[i])) {
             i++;
@@ -95,7 +95,7 @@ LinearGradientData resolve_linear_gradient_data(Layout::Node const& node, Gfx::F
     };
     while (i < color_stop_list.size() - 1) {
         auto& stop = color_stop_list[i];
-        if (!stop.color_stop.length.has_value()) {
+        if (!stop.color_stop.position.has_value()) {
             auto run_start = i - 1;
             auto start_position = resolved_color_stops[i++].transition_hint.value_or(resolved_color_stops[run_start].position);
             auto run_end = find_run_end();