As one of those people who complained about the HTML output of DocBook recently, it’s good to see Shaun kickstarting some progress on this.
I think my main concern with those mockups is the line length… in English, ten words is about the most you can fit on a line before reading speed slows down. This isn’t so much of an issue for stuff that turns up in help browsers, as the narrow width of the window naturally restricts this anyway, but it’s a problem for things like the HIG that are normally read in a web browser. I’ve tried to deal with this in the draft HIG’s stylesheet recently by indenting the body text from both sides of the page, which also makes it easier to pick out headers. (Example)
My main gripe with the HTML version of large DocBook documents, though, is the lack of a navigation sidebar. Things like the HIG are a pain in the bahookie to jump around, when every other major online styleguide (Mac, WIndows, Java) makes it simple.