LibWeb: Implement the CSS revert keyword

This is a universal value like `initial` and `inherit` and works by
reverting the current value to whatever we had at the start of the
current cascade origin.

The implementation is somewhat inefficient as we make a copy of all
current values at the start of each origin. I'm sure we can come up with
a way to make this faster eventually.

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

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/StyleValue.h>
+
+namespace Web::CSS {
+
+class RevertStyleValue final : public StyleValueWithDefaultOperators<RevertStyleValue> {
+public:
+    static ErrorOr<ValueComparingNonnullRefPtr<RevertStyleValue>> the()
+    {
+        static ValueComparingNonnullRefPtr<RevertStyleValue> instance = TRY(adopt_nonnull_ref_or_enomem(new (nothrow) RevertStyleValue));
+        return instance;
+    }
+    virtual ~RevertStyleValue() override = default;
+
+    ErrorOr<String> to_string() const override { return "revert"_string; }
+
+    bool properties_equal(RevertStyleValue const&) const { return true; }
+
+private:
+    RevertStyleValue()
+        : StyleValueWithDefaultOperators(Type::Revert)
+    {
+    }
+};
+
+}