LibWeb: Implement CSS unset builtin value

This is equivalent to `initial` or `inherit`, depending on if the
property is inherited by default.

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

@@ -1407,7 +1407,9 @@ RefPtr<StyleValue> Parser::parse_builtin_value(ParsingContext const&, StyleCompo
             return InheritStyleValue::the();
         if (ident.equals_ignoring_case("initial"))
             return InitialStyleValue::the();
-        // FIXME: Implement `unset` keyword
+        if (ident.equals_ignoring_case("unset"))
+            return UnsetStyleValue::the();
+        // FIXME: Implement `revert` and `revert-layer` keywords, from Cascade4 and Cascade5 respectively
     }
 
     return {};

Userland/Libraries/LibWeb/CSS/StyleProperties.cpp

@@ -71,6 +71,13 @@ Optional<NonnullRefPtr<StyleValue>> StyleProperties::property(CSS::PropertyID id
         return fetch_initial(id);
     if (value->is_inherit())
         return fetch_inherited(id);
+    if (value->is_unset()) {
+        if (is_inherited_property(id)) {
+            return fetch_inherited(id);
+        } else {
+            return fetch_initial(id);
+        }
+    }
 
     return value;
 }

Userland/Libraries/LibWeb/CSS/StyleValue.h

@@ -219,6 +219,7 @@ public:
         Invalid,
         Inherit,
         Initial,
+        Unset,
         String,
         Length,
         Color,
@@ -245,6 +246,7 @@ public:
 
     bool is_inherit() const { return type() == Type::Inherit; }
     bool is_initial() const { return type() == Type::Initial; }
+    bool is_unset() const { return type() == Type::Unset; }
     bool is_color() const { return type() == Type::Color; }
     bool is_identifier() const { return type() == Type::Identifier || is_auto(); }
     bool is_image() const { return type() == Type::Image; }
@@ -266,11 +268,11 @@ public:
     bool is_overflow() const { return type() == Type::Overflow; }
     bool is_text_decoration() const { return type() == Type::TextDecoration; }
 
-    bool is_builtin() const { return is_inherit() || is_initial(); }
+    bool is_builtin() const { return is_inherit() || is_initial() || is_unset(); }
 
     bool is_builtin_or_dynamic() const
     {
-        return is_inherit() || is_initial() || is_custom_property() || is_calculated();
+        return is_builtin() || is_custom_property() || is_calculated();
     }
 
     virtual String to_string() const = 0;
@@ -568,6 +570,24 @@ private:
     }
 };
 
+class UnsetStyleValue final : public StyleValue {
+public:
+    static NonnullRefPtr<UnsetStyleValue> the()
+    {
+        static NonnullRefPtr<UnsetStyleValue> instance = adopt_ref(*new UnsetStyleValue);
+        return instance;
+    }
+    virtual ~UnsetStyleValue() override { }
+
+    String to_string() const override { return "unset"; }
+
+private:
+    UnsetStyleValue()
+        : StyleValue(Type::Unset)
+    {
+    }
+};
+
 class ColorStyleValue : public StyleValue {
 public:
     static NonnullRefPtr<ColorStyleValue> create(Color color)