LibWeb: Implement CSSStyleDeclaration.{set,remove}Property close to spec

We already had setProperty() but it was full of ad-hoc idiosyncracies.
This patch aligns setProperty() with the CSSOM spec and also implements
removeProperty() since that's actually needed by setProperty() now.

Some things fixed by this:
- We now support the "priority" parameter to setProperty()
- Element "style" attributes now update to reflect CSSOM mutations

Userland/Libraries/LibWeb/CSS/ResolvedCSSStyleDeclaration.cpp

@@ -800,9 +800,18 @@ Optional<StyleProperty> ResolvedCSSStyleDeclaration::property(PropertyID propert
     };
 }
 
-bool ResolvedCSSStyleDeclaration::set_property(PropertyID, StringView)
+// https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-setproperty
+DOM::ExceptionOr<void> ResolvedCSSStyleDeclaration::set_property(PropertyID, StringView, StringView)
 {
-    return false;
+    // 1. If the computed flag is set, then throw a NoModificationAllowedError exception.
+    return DOM::NoModificationAllowedError::create("Cannot modify properties in result of getComputedStyle()");
+}
+
+// https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-removeproperty
+DOM::ExceptionOr<String> ResolvedCSSStyleDeclaration::remove_property(PropertyID)
+{
+    // 1. If the computed flag is set, then throw a NoModificationAllowedError exception.
+    return DOM::NoModificationAllowedError::create("Cannot remove properties from result of getComputedStyle()");
 }
 
 String ResolvedCSSStyleDeclaration::serialized() const