BlueGriffon: a cross-platform WYSIWYG HTML5 and CSS3 editor based on Firefox 4
BlueGriffon is built upon a recent version of Firefox 4. It uses the same Gecko page layout engine -- and in fact, lots of the UI elements look like they've come straight from Firefox 4. There are even add-ons to provide extra functionality, but they'll cost you 30 euros (about $40).
As far as actual usability goes, BlueGriffon is pretty good. The UI isn't exactly easy to use (but these things rarely are), but source editor is good -- and there's a built-in SVG creator that works well. You have access to all of the HTML5 tag types and CSS3 selectors, but not through the right-click context menu, which is annoying. The primary feature of BlueGriffon has to be the ability to see the effect of CSS3 selectors instantly -- or simply as a text editor that has a built-in Firefox 4 preview window.
According to WebUpd8, the nightly builds are the most stable. Also, if you're using Linux, grab the binary installer rather than the .deb, which apparently has some issues. The Windows build crashed quite a few times during my tests, too.