HTML5 is a brand new (well, not so new) form of HTML that is growing in popularity every day. All the modern browsers tend to support rendering HTML5 with no problems. It is widely the optimal choice for smart phone browsers and more and more sites are adopting it.
But what about GoogleBot? Can Google crawl HTML5? Does Google pick up any additional factors from HTML5 to give your site a boost in the search results? Can it hurt your SEO efforts?
My take aways are (1) Google currently won't give special consideration to those that use HTML5 and (2) it might not be worth even going there, if you are doing it for SEO reasons. To me, it seems John is implying to shy away from it, but he knows how awesome HTML5 can be for users, but how awesome for crawlers?
Let me quote John's two posts, they are similar:
In general, our crawlers are used to not being able to parse all HTML markup - be it from broken HTML, embedded XML content or from the new HTML5 tags. Our general strategy is to wait to see how content is marked up on the web in practice and to adapt to that. If we find that more and more content uses HTML5 markup, that this markup can give us additional information, and that it doesn't cause problems if webmasters incorrectly use it (which is always a problem in the beginning), then over time we'll attempt to work that into our algorithms. With that in mind, I definitely wouldn't want to stand in the way of your implementing parts of your site with HTML5, but I also wouldn't expect to see special treatment of your content due to the HTML5 markup at the moment. HTML5 is still very much a work in progress, so it's great to see bleeding-edge sites making use of the new possibilities :)
In general, we work hard to understand as much of the web as possible, but I have a feeling that HTML5 markup is not yet as widely in use (and in use correctly) that it would make sense for us to use it as a means of understanding content better. As HTML5 gains in popularity and as we recognize specific markup elements that provide value to our indexing system, this is likely to change, but at the moment I would not assume that you would have an advantage by using HTML5 instead of older variants.
Returning different content to search engine crawlers as you would to similar users would be considered cloaking and against our Webmaster Guidelines. I would not recommend treating search engine crawlers different than you would similar users.
Personally, I would recommend using HTML5 where you think that it already makes sense, perhaps reverting to HTML4 if you can determine that the browser won't support the elements of HTML5 that you use properly. While this will not result in an advantage for your content in our search results, it generally wouldn't be disadvantageous either.