There are new technologies coming out on the web all the time and Google isn’t always up to date in understanding the markup used on those web pages.
So what does Google do when it doesn’t understand the markup? It often ignore the markup and just looks at the text in or around the markup.
Google’s John Mueller was asked this in a Google Webmaster Help thread in which he responded:
In cases where we don’t understand the markup that’s used, we’ll generally ignore the tag and just keep the text. That allows us to keep up with most of these changes, be it from XML pages or new HTML5 tags like these. A quick way to double-check is to look at the text-version of a cached page.
Now, this doesn’t mean you should make up new markup languages and stuff keywords in them. Don’t do that!
Forum discussion at Google Webmaster Help.