"but on the other hand as an open source project anyone can clone its code and continue working on it and with that in mind i wouldn't say its easy to kill (but not imposible)."
that is not an criteria, if i create an 0 bytes text document, and distribute it online, techinicaly its an open source document that does nothing.
any one can download and add new features and give it an purpose, but that dont means its alive, if it dont have maintainers, for all intentes and purposes its dead.
any one can fork it, but having too many programs (forks) with the same name maybe confusing and the name may end up changint to avoid harm the reputation if someone make an malicious code with the name. (so the name may be an brand copyrighted)
if you cant add new features yourself, take a look at the github and click on the number at the right of "forks" see what fork is active, and as long as there is at least 1, its alive.
the oficial branch may acept patches from it on the future or not, in the mean time you can use one of the forks.
"HTML5/Browser environment is ever-changing what works today may generate error tomorrow....."
nope, the specs for html5 has been complete, now w3c is working on html6 i guess.
i any case, as long as they didnt use something in draft stage, things should work forever.
HTML will get new features but the old ones will keep working (except the draft ones as i said before)