I believe the injecting script part is to support having the chrome and the content in different processes.<p>I&#x27;m sad that, on the UI modification front, the new system doesn&#x27;t seem nearly as powerful as XUL overlays. I worry that when Servo comes into production use, all that extensibility will be dropped (with Chrome not having it as the justification) and we&#x27;ll be left without a highly customizable mainstream browser.