LibWeb: Implement HTMLStyleElement.sheet

Author: https://github.com/Lubrsi
Commit: 10d4f2fc1e
Pull-request: https://github.com/SerenityOS/serenity/pull/10282
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/HTML/HTMLStyleElement.idl

@@ -4,4 +4,7 @@ interface HTMLStyleElement : HTMLElement {
 
     [Reflect] attribute DOMString type;
 
+    // FIXME: This should come from a LinkStyle mixin
+    readonly attribute CSSStyleSheet? sheet;
+
 };