That's really quite debatable.
AtheOS has a KHTML port, the guy that wrote atheOS and the KHTML port....pure genius, not many like him.
But the AtheOS team has decided going forward in the future to port the gecko engine as they claim it is more portable.
Fact is, the gecko engine, separated out from the mozilla app...which btw, mozilla is eol already....but the gecko engine is quite portable and a very competent rendering engine...and I believe this contest has said phoenix, firebird, or some gecko based browser would be suficient to win the contest.
No real need to change the rules, though KHTML would be very nice to have...in fact one person said he's already working on it...
I'm working on doing a browser myself...taking my sweet time about it...but nevertheless...I am working on a browser.
I'm doing a much simpler port of an earlier rendering engine that won't support css, though it would support javascipt and frames....
ahhh, maybe I'd join someone elses effort, assuming they get somewhere and I get somewhere and it makes sense.