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:

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.