LibWeb: Add preliminary support for CSS animations

This partially implements CSS-Animations-1 (though there are references
to CSS-Animations-2).
Current limitations:
- Multi-selector keyframes are not supported.
- Most animation properties are ignored.
- Timing functions are not applied.
- Non-absolute values are not interpolated unless the target is also of
  the same non-absolute type (e.g. 10% -> 25%, but not 10% -> 20px).
- The JavaScript interface is left as an exercise for the next poor soul
  looking at this code.

With those said, this commit implements:
- Interpolation for most common types
- Proper keyframe resolution (including the synthetic from-keyframe
  containing the initial state)
- Properly driven animations, and proper style invalidation

Co-Authored-By: Andreas Kling <kling@serenityos.org>

Userland/Libraries/LibWeb/CSS/CSSKeyframeRule.h

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2023, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <LibWeb/CSS/CSSRule.h>
+#include <LibWeb/CSS/CSSStyleDeclaration.h>
+#include <LibWeb/CSS/Percentage.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/WebIDL/ExceptionOr.h>
+
+namespace Web::CSS {
+
+// https://drafts.csswg.org/css-animations/#interface-csskeyframerule
+class CSSKeyframeRule final : public CSSRule {
+    WEB_PLATFORM_OBJECT(CSSKeyframeRule, CSSRule);
+
+public:
+    static WebIDL::ExceptionOr<JS::NonnullGCPtr<CSSKeyframeRule>> create(JS::Realm& realm, CSS::Percentage key, CSSStyleDeclaration& declarations)
+    {
+        return MUST_OR_THROW_OOM(realm.heap().allocate<CSSKeyframeRule>(realm, realm, key, declarations));
+    }
+
+    virtual ~CSSKeyframeRule() = default;
+
+    virtual Type type() const override { return Type::Keyframe; };
+
+    CSS::Percentage key() const { return m_key; }
+    JS::NonnullGCPtr<CSSStyleDeclaration> style() const { return m_declarations; }
+
+private:
+    CSSKeyframeRule(JS::Realm& realm, CSS::Percentage key, CSSStyleDeclaration& declarations)
+        : CSSRule(realm)
+        , m_key(key)
+        , m_declarations(declarations)
+    {
+    }
+
+    virtual void visit_edges(Visitor&) override;
+    virtual JS::ThrowCompletionOr<void> initialize(JS::Realm&) override;
+    virtual DeprecatedString serialized() const override;
+
+    CSS::Percentage m_key;
+    JS::NonnullGCPtr<CSSStyleDeclaration> m_declarations;
+};
+
+template<>
+inline bool CSSRule::fast_is<CSSKeyframeRule>() const { return type() == CSSRule::Type::Keyframe; }
+
+}