I presume part of this is that for a long while the KDE/webkit/chrome engine was a separate, isolated thing, whereas the rendering in Firefox was not. Not sure

I presume part of this is that for a long while the KDE/webkit/chrome engine was a separate, isolated thing, whereas the rendering in Firefox was not. Not sure where Firefox/gecko is now, but pulling the rendering apart from the UI and improving it to be usable by other UIs, projects, would help get this done.