CSS of KHTML never implemented the full standard, now it's totally outdated. I don't see a way to get it on track again - and I doubt it would be of any use. KHTML is good for browsing the man:// and info:// pages - there's no better way to do it.
I don't think the major problem is CSS; the major problem must be JavaScript and the different APIs which are constantly increasing in number (WebAudio, WebSockets, there was even an API for accessing USB devices if I remember correctly) and all this is just adding to the bloat of the "modern" Web. Does one /really/ need all of these APIs? What's the next step? An API for brewing coffee?
In this frustrating course that the Web has taken it does not surprise me how many people are "going back to the roots" and re-discovering Gopher just for fun... There is even this new protocol, Gemini, which was created because the Web is the mess it is.
I think it is highly improbable that KHTML will thrive in its current form. Speaking historically, WebKit was based on KHTML, so why not port WebKit instead? There is no way that such a small team be able to compete with giants like Google and... ehh... Google. I don't know about Mozilla. They give away an impression of stagnation. Nothing seems to truly change except for the constant UI reorganizations and option removal. They even threw away their "native" extensions API for the disfunctional WebExtensions solution.
-- Mavridis Philippe