Now, the Kindle file already included support for a pair of HTML5 tags, audio and video. The Kindle itself cannot support these tags, but you can use them to embed audio and video files that can be read by the Kindle iOS apps. The new file spec includes support for a number of formatting features that I've been wanting for a long time now as well as a number of rather exotic components that I'm not sure any one is using, even in Epub.
It's important to remember that at this point we don't know quite how Amazon will use the tags nor do we know which of the tags will actually end up inside the Kindle ebook file itself.
You see, Amazon has long allowed you to make ebooks using the latest valid HTML 4 tags. Amazon then took the code you supplied, threw out the formatting that wasn't supported, and made an ebook which contained the old, funky, and limited formatting supported by the current Kindle (and old Mobipocket) apps.
Trust me, I've looked at the actual contents of a Kindle ebook. There is often a big difference between the source content and the ebook. In fact, the HTML5 tags that Amazon currently use in the Kindle format are not used according to spec. Amazon did their own funky thing with the tags, and I suspect they will repeat that behavior.
Given the vast change in the supported tags, I wonder if Amazon will also make a radical change to the Kindle file itself?
Right now the standard Kindle ebook is a single file with metadata at the front, followed by one long string of text (the content of the ebook, TOC,and more), with images attached to the end. The file is often compressed, but the organization never changes.
Here's the interesting part. If you want to support really complex formatting, it's best to have multiple files. That's how Epub does it; the Epub file itself is actually a ZIP file. If you unzip it you will see that it contains a bunch of different files and folders. Amazon will likely have to do the same thing with the new Kindle ebook file. Oh, joy.
BTW, you might want to replace your Kindle 2 if you still have one. The various Kindles will need a firmware update before they can support the new format, and there's almost no chance that Amazon will update the K2 again.