LibWeb: Add URLStyleValue to represent general url() values

This is primarily being added to support `fill: url(#gradient)` for
SVGs.

Userland/Libraries/LibWeb/CSS/StyleValue.cpp

@@ -51,6 +51,7 @@
 #include <LibWeb/CSS/StyleValues/TextDecorationStyleValue.h>
 #include <LibWeb/CSS/StyleValues/TimeStyleValue.h>
 #include <LibWeb/CSS/StyleValues/TransformationStyleValue.h>
+#include <LibWeb/CSS/StyleValues/URLStyleValue.h>
 #include <LibWeb/CSS/StyleValues/UnresolvedStyleValue.h>
 #include <LibWeb/CSS/StyleValues/UnsetStyleValue.h>
 
@@ -325,6 +326,12 @@ UnsetStyleValue const& StyleValue::as_unset() const
     return static_cast<UnsetStyleValue const&>(*this);
 }
 
+URLStyleValue const& StyleValue::as_url() const
+{
+    VERIFY(is_url());
+    return static_cast<URLStyleValue const&>(*this);
+}
+
 StyleValueList const& StyleValue::as_value_list() const
 {
     VERIFY(is_value_list());

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -131,6 +131,7 @@ public:
         Transformation,
         Unresolved,
         Unset,
+        Url,
         ValueList
     };
 
@@ -180,6 +181,7 @@ public:
     bool is_transformation() const { return type() == Type::Transformation; }
     bool is_unresolved() const { return type() == Type::Unresolved; }
     bool is_unset() const { return type() == Type::Unset; }
+    bool is_url() const { return type() == Type::Url; }
     bool is_value_list() const { return type() == Type::ValueList; }
 
     bool is_builtin() const { return is_inherit() || is_initial() || is_unset(); }
@@ -228,6 +230,7 @@ public:
     TransformationStyleValue const& as_transformation() const;
     UnresolvedStyleValue const& as_unresolved() const;
     UnsetStyleValue const& as_unset() const;
+    URLStyleValue const& as_url() const;
     StyleValueList const& as_value_list() const;
 
     AbstractImageStyleValue& as_abstract_image() { return const_cast<AbstractImageStyleValue&>(const_cast<StyleValue const&>(*this).as_abstract_image()); }
@@ -274,6 +277,7 @@ public:
     TransformationStyleValue& as_transformation() { return const_cast<TransformationStyleValue&>(const_cast<StyleValue const&>(*this).as_transformation()); }
     UnresolvedStyleValue& as_unresolved() { return const_cast<UnresolvedStyleValue&>(const_cast<StyleValue const&>(*this).as_unresolved()); }
     UnsetStyleValue& as_unset() { return const_cast<UnsetStyleValue&>(const_cast<StyleValue const&>(*this).as_unset()); }
+    URLStyleValue& as_url() { return const_cast<URLStyleValue&>(const_cast<StyleValue const&>(*this).as_url()); }
     StyleValueList& as_value_list() { return const_cast<StyleValueList&>(const_cast<StyleValue const&>(*this).as_value_list()); }
 
     bool has_auto() const;

Userland/Libraries/LibWeb/CSS/StyleValues/URLStyleValue.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2023, MacDue <macdue@dueutil.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/URL.h>
+#include <LibWeb/CSS/Serialize.h>
+#include <LibWeb/CSS/StyleValue.h>
+
+namespace Web::CSS {
+
+class URLStyleValue final : public StyleValueWithDefaultOperators<URLStyleValue> {
+public:
+    static ValueComparingNonnullRefPtr<URLStyleValue> create(AK::URL const& url)
+    {
+        return adopt_ref(*new URLStyleValue(url));
+    }
+
+    virtual ~URLStyleValue() override = default;
+
+    AK::URL const& url() const { return m_url; }
+
+    bool properties_equal(URLStyleValue const& other) const { return m_url == other.m_url; }
+
+    virtual ErrorOr<String> to_string() const override
+    {
+        return serialize_a_url(m_url.to_deprecated_string());
+    }
+
+private:
+    URLStyleValue(AK::URL const& url)
+        : StyleValueWithDefaultOperators(Type::Url)
+        , m_url(url)
+    {
+    }
+
+    AK::URL m_url;
+};
+
+}

Userland/Libraries/LibWeb/Forward.h

@@ -155,6 +155,7 @@ class TransformationStyleValue;
 class UnicodeRange;
 class UnresolvedStyleValue;
 class UnsetStyleValue;
+class URLStyleValue;
 
 enum class MediaFeatureID;
 enum class PropertyID;