Reject "text" value in background shorthand.

components/style/properties/shorthand/background.mako.rs

@@ -69,6 +69,13 @@
                     if ${name}.is_none() {
                         if let Ok(value) = input.try(|input| background_${name}::single_value
                                                                                ::parse(context, input)) {
+                            % if name == "clip" and product == "gecko":
+                            // "text" value of background-clip should not be part of background
+                            // shorthand per current spec and impls.
+                            if value == background_clip::single_value::SpecifiedValue::text {
+                                return Err(());
+                            }
+                            % endif
                             ${name} = Some(value);
                             continue
                         }