Yes, in the alpha version of KM1.6.0a3 it is already supported.
I really don't know anything about HTML5, I just know that with the new KM1.6 you can watch video using the new html tag.
html5.enable userset boolean true
html5.flushtimer.initialdelay userset integer 120
html5.flushtimer.subsequentdelay userset integer 120
html5.offmainthread userset boolean true
.. those settings have no affect in gecko 1.8 nor will they change how websites supporting html5 will render.
if it works in video websites that require html5, that's just because the website has detected no videotag support and playing either in flash or parsing through java container