When it comes to figuring out what browsers supported HTML5 and when they supported it you discover things are grey as grey can get.
Various browsers initially supported features that made it into the HTML5 specification which included the likes of even IE6-IE8, but this support represented a comparably limited subset of the available features in the current version of HTML5 (thus these browsers scored rather low scores in HTML5 browser tests).
2008 is when HTML5 started seeing its first semi-official support implemented.
Safari 3.1 (March 2008), Opera 9.5 (June 2008) had introductory support for the standard under the title of being HTML5. Firefox 2.0 had some limited support but they never announced it as HTML5 support but they included a few additions that were some of the key areas of HTML5.
Opera 9.6 (June 2008) / Safari 3.2 (November 2008) / Firefox 3.0 (June 2008) all extended support for these features too. Opera 9.6 also introduced HTML5 audio support in a limited form.
By the time Chrome hit version 2.0 (January 2009) it had a fair subset of HTML5 support in this release, but this support paled compared to its current support for the standard.
With the release of Firefox 3.5 though (June 2009) HTML5 Video and Audio tags were supported and helped HTML5 'go mainstream' as they announced it to all new users of the browser. Likewise with Safari 4 (June 2009) HTML5 support was greatly enhanced and video/audio support was also added. Opera 10 in June 2009 was the highest rated HTML5 supporting browser in browser tests but had no initial HTML5 video support till Opera 10.6 (Late 2009).
Chrome continued to throughout its releases grow increasing support for HTML5 at a rapid rate (so it's harder to pin point specific support) but by Chrome 6 (May 2010) it had gained HTML5 video/audio support and by Chrome 8 (October 2010) there was already support for all but two of the key elements of HTML5.
The first official version of IE to support HTML5 (despite having had some incidental support since IE7/8 days) was IE9 which only hit final release in March 2011.
So as things have progressed there has been browsers with significant support for HTML5 since mid-2009 (with the exception of IE which didn't hit the market with proper support till this year). With Safari 4.0, Opera 10.6, Firefox 3.5, Chrome 8 browsers onwards all offering support representing the majority of the HTML5 standard. HTML5 support continues to evolve across all mainstream browsers.
Those interested in learning more can check out a few references to see more details on this subject:
Browser Market Share:
For those interested in the approximate market share of browsers offering significant degrees of HTML5 support as of August 2011 the stats site show:
W3C Counter: Supporting browsers 54.38%~
Firefox 3.6+ - 24%~
IE 9 - 6.53%
Chrome 12+ - 18.52%
Safari 5 - 5.33%
W3Schools: Supporting browsers 78.7%~
IE 9 - 4.2%
Firefox 3.5+ - 39.6%
Chrome 8+ - 29.3%
Safari 4+ - 3.8%
Opera 10+ - 1.8%
StatCounter: Supporting browsers 55.3%~
Firefox 3.5+ - 22%~
Chrome 8+ - 23%~
Safari 5 - 2.25%
IE9 - 8.05%
StatOwl: Supporting browsers 57.09%~
Firefox 3.x - 8%~
Firefox 4+ - 14%~
Opera 11 - 0.32%
IE9 - 10.7%
Chrome 9+ - 13.37%
Safari 4+ -10.7%~
NetApplications: Supporting browsers 52.75%~
Firefox 3.5+ - 23%~
Chrome 8+ - 16%~
Opera 10.x+ - 1.54%
IE9 - 7.91%
Safari 4.0+ - 4.3%~