WebRender is not coupled in any way to HTML/CSS/JS as far as I know (I've used it in an app).
Fundamentally, it allows you to represent a scene-graph and render it efficiently. How you go about constructing and manipulating your scene graph is up to the application which uses it.
In Servo's case it parses HTML and CSS for structure and style and allows manipulations through JS through the DOM, but none of those are any concern for webrender.