LibWeb/CSS: Add alternative src() syntax for URLs

url() has some limitations because of allowing unquoted URLs as its
contents. For example, it can't use `var()`. To get around this, there's
an alternative `src()` function which behaves the same as `url()` except
that it is parsed as a regular function, which makes `var()` and friends
work properly.

There's no WPT test for this as far as I can tell, so I added our own.

Libraries/LibWeb/CSS/Parser/ValueParsing.cpp

@@ -2710,7 +2710,6 @@ Optional<URL> Parser::parse_url_function(TokenStream<ComponentValue>& tokens)
     // <url> = <url()> | <src()>
     // <url()> = url( <string> <url-modifier>* ) | <url-token>
     // <src()> = src( <string> <url-modifier>* )
-    // FIXME: Also parse src() function
     auto transaction = tokens.begin_transaction();
     auto const& component_value = tokens.consume_a_token();
 
@@ -2721,7 +2720,17 @@ Optional<URL> Parser::parse_url_function(TokenStream<ComponentValue>& tokens)
     }
 
     // <url()> = url( <string> <url-modifier>* )
+    // <src()> = src( <string> <url-modifier>* )
+    if (component_value.is_function()) {
+        URL::Type function_type;
         if (component_value.is_function("url"sv)) {
+            function_type = URL::Type::Url;
+        } else if (component_value.is_function("src"sv)) {
+            function_type = URL::Type::Src;
+        } else {
+            return {};
+        }
+
         auto const& function_values = component_value.function().value;
         TokenStream url_tokens { function_values };
 
@@ -2802,7 +2811,7 @@ Optional<URL> Parser::parse_url_function(TokenStream<ComponentValue>& tokens)
         });
 
         transaction.commit();
-        return URL { url_string.token().string().to_string(), move(request_url_modifiers) };
+        return URL { url_string.token().string().to_string(), function_type, move(request_url_modifiers) };
     }
 
     return {};

Libraries/LibWeb/CSS/URL.cpp

@@ -10,8 +10,9 @@
 
 namespace Web::CSS {
 
-URL::URL(String url, Vector<RequestURLModifier> request_url_modifiers)
-    : m_url(move(url))
+URL::URL(String url, Type type, Vector<RequestURLModifier> request_url_modifiers)
+    : m_type(type)
+    , m_url(move(url))
     , m_request_url_modifiers(move(request_url_modifiers))
 {
 }
@@ -19,9 +20,18 @@ URL::URL(String url, Vector<RequestURLModifier> request_url_modifiers)
 // https://drafts.csswg.org/cssom-1/#serialize-a-url
 String URL::to_string() const
 {
-    // To serialize a URL means to create a string represented by "url(", followed by the serialization of the URL as a string, followed by ")".
+    // To serialize a URL means to create a string represented by "url(", followed by the serialization of the URL as a
+    // string, followed by ")".
+    // AD-HOC: Serialize as src() if it was declared as that.
     StringBuilder builder;
+    switch (m_type) {
+    case Type::Url:
         builder.append("url("sv);
+        break;
+    case Type::Src:
+        builder.append("src("sv);
+        break;
+    }
     serialize_a_string(builder, m_url);
 
     // AD-HOC: Serialize the RequestURLModifiers

Libraries/LibWeb/CSS/URL.h

@@ -44,7 +44,12 @@ private:
 // https://drafts.csswg.org/css-values-4/#urls
 class URL {
 public:
-    URL(String url, Vector<RequestURLModifier> = {});
+    enum class Type : u8 {
+        Url,
+        Src,
+    };
+
+    URL(String url, Type = Type::Url, Vector<RequestURLModifier> = {});
 
     String const& url() const { return m_url; }
     Vector<RequestURLModifier> const& request_url_modifiers() const { return m_request_url_modifiers; }
@@ -53,6 +58,7 @@ public:
     bool operator==(URL const&) const;
 
 private:
+    Type m_type;
     String m_url;
     Vector<RequestURLModifier> m_request_url_modifiers;
 };

Tests/LibWeb/Text/expected/css/src-function.txt

@@ -0,0 +1,5 @@
+Before: none
+Using url('cool.png'): url("cool.png")
+Using url(var(--some-url)): none
+Using src('cool.png'): src("cool.png")
+Using src(var(--some-url)): src("awesome.png")

Tests/LibWeb/Text/input/css/src-function.html

@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<script src="../include.js"></script>
+<div id="target" style="--some-url: 'awesome.png'"></div>
+<script>
+    test(() => {
+        let target = document.getElementById("target");
+        println(`Before: ${getComputedStyle(target).backgroundImage}`);
+
+        target.style.backgroundImage = "url('cool.png')";
+        println(`Using url('cool.png'): ${getComputedStyle(target).backgroundImage}`);
+
+        target.style.backgroundImage = "";
+        target.style.backgroundImage = "url(var(--some-url))";
+        println(`Using url(var(--some-url)): ${getComputedStyle(target).backgroundImage}`);
+
+        target.style.backgroundImage = "";
+        target.style.backgroundImage = "src('cool.png')";
+        println(`Using src('cool.png'): ${getComputedStyle(target).backgroundImage}`);
+
+        target.style.backgroundImage = "";
+        target.style.backgroundImage = "src(var(--some-url))";
+        println(`Using src(var(--some-url)): ${getComputedStyle(target).backgroundImage}`);
+
+    });
+</script>