Though they don't really mention it in the post, they use the new Local Storage APIs (which were a reification of Google Gears) to programmatically cache both the application and the user's data, at least on the iPhone version.

In the article they're mostly using 'HTML5' to mean 'not flash' and 'not native'.

I think the proper name for the HTML5 feature they're using is the "application cache", which caches all the files listed in the manifest etc.

I'm pretty sure they're also using the Sqlite3-based "Local Storage" API for locally caching your mail data.

Note that "HTML5" is in the title of this series of blog posts. On the other hand, the same criticism has been levelled at some other entries in the series too.

But you can't really have HTML5 without HTML, just as you might write about C tricks in a Objective-C or Cocoa article.