I believe the injecting script part is to support having the chrome and the content in different processes. I'm sad that, on the UI modification front, the new

I believe the injecting script part is to support having the chrome and the content in different processes. I'm sad that, on the UI modification front, the new system doesn'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'll be left without a highly customizable mainstream browser.