LibWeb: Port the CSS namespace to IDL

Author: https://github.com/trflynn89
Commit: 2d45e1fca5
Pull-request: https://github.com/SerenityOS/serenity/pull/17870
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibWeb/CSS/CSS.idl

@@ -0,0 +1,8 @@
+// https://www.w3.org/TR/cssom-1/#namespacedef-css
+[Exposed=Window]
+namespace CSS {
+    CSSOMString escape(CSSOMString ident);
+
+    boolean supports(CSSOMString property, CSSOMString value);
+    boolean supports(CSSOMString conditionText);
+};