Last Thursday I gave a techtalk [1] at Liip Zurich about which html 5 features are supported by what (modern) browser, incl. my personal recommendation what's worth following and what maybe not yet.
The result is this spreadsheet and these slides (but they are basically just the spreadsheet spread across many pages :))

The table needs some explanation. The first few columns (the browsers) should be clear, gears hopefully, too. The "Flash/JS Fallback" states, if there is a possible and feasible Fallback in Flash or Javascript.

The "Usable with latest?" column summarizes, if that feature is available in Firefox 3.5, Safari 4 and Internet Explorer 8 (sorry, no Opera here). This is under the assumption, that if this feature is supported by those three browsers, that it will actually have some market share in a few years (...). Anything else (basically meaning anything which is not supported by IE8 now) will have a hard time in the "shorter" term.

WIth "Useable with decent degration?" I wanted to state, if you could use that feature for people with modern browsers, but still support people without (maybe with less features). Without having to go through hoops in your code, eg. Native JSON is pretty easy to support.

The last column "Recommended in a 'controlled' environment (IMHO)" is just my humble opinion. It rates each feature, if it is worth to develop for/with, if you can somehow control (or know) your environment. This column assumes, that your audience is either using a modern browser or is able to install Google Gears. Still, to get a "green" by me, there has to be a decent fallback, even if that fallback is just to properly *not* support it (meaning that it doesn't hinder people using the interface without having the support for that feature).

As I said, the last 2-3 columns are my personal opinion (either the content or how I assembled it), which may differ from yours. So I'm really interested in getting feedback. I still hope it may be useful for you, I certainly was dearly missing such an overview and getting all the facts together was quite a task. I will also apply new findings to the table in the future

[1] Techtalks are a weekly institution at Liip, where a Liipie (or someone else) talks about something interesting for half an hour just before lunch.

TwoHawks @ 14.07.2009 17:19 CET
Thank you for this investigative push and the charting, Christian. I hope we see a lot more of this kind of info, esp including more of the mundane feature changes. Can't wait to see what others have to say about it.

Just to note, I came here this morning searching on
"html 5" multi "column layout"
for those of us out there working on complex 3+ column layouts sans JS, with sticky headers and footers I am anxious to see what the cutting edge HTML-5 crowd is coming up with.
sorry I could not offer more.
TwoHawks
Stateline, NV, usa