While most websites have transitioned to using the mobile friendly HTML5 programming language, some sites still use Flash. Google’s making this slightly less frustrating by warning users of pages that use Flash. The warnings show up in Google’s search results and look like this:
Google’s algorithms are smart enough to detect how much Flash a site uses and may not flag sites that make minimal use of it.
The company is using this as an opportunity to push web developers to use HTML5 “since it is universally supported, sometimes exclusively, by all devices.”
Source: Google Webmaster Central Blog