LibWeb: Implement CSSStyleDeclaration.cssText

Author: https://github.com/Lubrsi
Commit: c247fefee7
Pull-request: https://github.com/SerenityOS/serenity/pull/15964
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/CSS/ResolvedCSSStyleDeclaration.cpp

@@ -570,4 +570,11 @@ String ResolvedCSSStyleDeclaration::serialized() const
     return String::empty();
 }
 
+// https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-csstext
+WebIDL::ExceptionOr<void> ResolvedCSSStyleDeclaration::set_css_text(StringView)
+{
+    // 1. If the computed flag is set, then throw a NoModificationAllowedError exception.
+    return WebIDL::NoModificationAllowedError::create(realm(), "Cannot modify properties in result of getComputedStyle()");
+}
+
 }