Mac version coming soon, and why would you want to support earlier than WinXP - well, I suppose Win2k would be potentially useful...
I think the comic is a good idea: it tries to explain what's gone into this and also explains why it might be seen as sluggish in some time instances and what unique approaches they've taken.
Amiga implications for classic OS? Sod all. There's no chance on earth anything remotely like this will make it across to classic. How about more modern stuff, or AROS? Dunno. Look at how far Rob got with the AROS browser... This is a lot more on top of webkit. Could it make it to OS4? Dunno, but again I'm doubtful and to me it's an irrelevance as, much is I'd like to run it, I can't buy hardware to run it!
But hats off to Google, this really is trying to innovate in the browser, ultimately for self serving purposes but at least they don't try to hide that!