LibWeb+LibGfx: Serialize HTML canvas fill/strokeStyle colors correctly

Before this change we were serializing them in a bogus 8-digit hex color
format that isn't actually recognized by HTML.

This code will need more work when we start supporting color spaces
other than sRGB.

(cherry picked from commit 4590c081c2eb257232463ed660498a02f9bbe7b8;
amended expected output because serenity does not yet have
LadybirdBrowser/ladybird#1603)

Tests/LibWeb/Text/expected/canvas/fillStyle-serialization.txt

@@ -0,0 +1,5 @@
+  `green` -> `#008000`
+`rgba(128, 128, 128, 0.4)` -> `rgba(128, 128, 128, 0.4)`
+`rgba(128, 128, 128, 0)` -> `rgba(128, 128, 128, 0)`
+`rgba(128, 128, 128, 1)` -> `#808080`
+`rgb(128, 128, 128)` -> `#808080`

Tests/LibWeb/Text/expected/canvas/fillstyle.txt

@@ -1,5 +1,5 @@
-1. "#00ff00ff"
-2. "#ff0000ff"
-3. "#0000ffff"
-4. "#00ff00ff"
-5. "#ff0000ff"
+1. "#00ff00"
+2. "#ff0000"
+3. "#0000ff"
+4. "#00ff00"
+5. "#ff0000"

Tests/LibWeb/Text/input/canvas/fillStyle-serialization.html

@@ -0,0 +1,18 @@
+<script src="../include.js"></script>
+<canvas id="c" width=300 height=300></canvas>
+<script>
+    test(() => {
+        let x = c.getContext("2d");
+
+        function go(color) {
+            x.fillStyle = color;
+            println("`" + color + "` -> `" + x.fillStyle + "`");
+        }
+
+        go("green");
+        go("rgba(128, 128, 128, 0.4)");
+        go("rgba(128, 128, 128, 0)");
+        go("rgba(128, 128, 128, 1)");
+        go("rgb(128, 128, 128)");
+    });
+</script>

Userland/Libraries/LibGfx/Color.cpp

@@ -18,9 +18,29 @@
 
 namespace Gfx {
 
-String Color::to_string() const
+String Color::to_string(HTMLCompatibleSerialization html_compatible_serialization) const
 {
-    return MUST(String::formatted("#{:02x}{:02x}{:02x}{:02x}", red(), green(), blue(), alpha()));
+    // If the following conditions are all true:
+
+    // 1. The color space is sRGB
+    // NOTE: This is currently always true for Gfx::Color.
+
+    // 2. The alpha is 1
+    // NOTE: An alpha value of 1 will be stored as 255 currently.
+
+    // 3. The RGB component values are internally represented as integers between 0 and 255 inclusive (i.e. 8-bit unsigned integer)
+    // NOTE: This is currently always true for Gfx::Color.
+
+    // 4. HTML-compatible serialization is requested
+    if (alpha() == 255
+        && html_compatible_serialization == HTMLCompatibleSerialization::Yes) {
+        return MUST(String::formatted("#{:02x}{:02x}{:02x}", red(), green(), blue()));
+    }
+
+    // Otherwise, for sRGB the CSS serialization of sRGB values is used and for other color spaces, the relevant serialization of the <color> value.
+    if (alpha() < 255)
+        return MUST(String::formatted("rgba({}, {}, {}, {})", red(), green(), blue(), alpha() / 255.0));
+    return MUST(String::formatted("rgb({}, {}, {})", red(), green(), blue()));
 }
 
 String Color::to_string_without_alpha() const

Userland/Libraries/LibGfx/Color.h

@@ -405,7 +405,12 @@ public:
         return m_value == other.m_value;
     }
 
-    String to_string() const;
+    enum class HTMLCompatibleSerialization {
+        No,
+        Yes,
+    };
+
+    [[nodiscard]] String to_string(HTMLCompatibleSerialization = HTMLCompatibleSerialization::No) const;
     String to_string_without_alpha() const;
 
     ByteString to_byte_string() const;

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

@@ -2850,6 +2850,7 @@ RefPtr<CSSStyleValue> Parser::parse_rgb_color_value(TokenStream<ComponentValue>&
             inner_tokens.skip_whitespace();
 
             alpha = parse_number_percentage_value(inner_tokens);
+
             inner_tokens.skip_whitespace();
 
             if (inner_tokens.has_next_token())

Userland/Libraries/LibWeb/HTML/Canvas/CanvasState.h

@@ -59,7 +59,7 @@ public:
         {
             return m_fill_or_stroke_style.visit(
                 [&](Gfx::Color color) -> JsFillOrStrokeStyle {
-                    return color.to_string();
+                    return color.to_string(Gfx::Color::HTMLCompatibleSerialization::Yes);
                 },
                 [&](auto handle) -> JsFillOrStrokeStyle {
                     return handle;