LibWeb: Implement CSSRule and CSSStyleDeclaration serialization

There are a handful of FIXME's here, but this seems generally good.
Note that CSS *values* don't get serialized in a spec-compliant way
since we currently rely on StyleValue::to_string() which is ad-hoc.

Userland/Libraries/LibWeb/CSS/CSSRule.cpp

@@ -12,4 +12,17 @@ CSSRule::~CSSRule()
 {
 }
 
+// https://drafts.csswg.org/cssom/#dom-cssrule-csstext
+String CSSRule::css_text() const
+{
+    // The cssText attribute must return a serialization of the CSS rule.
+    return serialized();
+}
+
+// https://drafts.csswg.org/cssom/#dom-cssrule-csstext
+void CSSRule::set_css_text(StringView)
+{
+    // On setting the cssText attribute must do nothing.
+}
+
 }