Google advising websites to use HTML5 instead of Flash for multimedia is nothing new. What is new is that it can directly cost you search rankings now.

As you’ve probably hears by now, Google announced a new ranking signal based on websites’ mobile-friendliness. This went into effect beginning earlier this week, though it may take a week or to to completely roll out.

There are a lot of reasons your webpages may register as non-mobile-friendly, but one that is sure to be a problem is the use of Flash.

“Avoid common mistakes that frustrate mobile visitors, such as featuring unplayable videos (e.g., Flash video as the page’s significant content),” Google tells webmasters. “Mobile pages that provide a poor searcher experience can be demoted in rankings or displayed with a warning in mobile search results.”

Google has of course been warning against this for years.

All the way back in 2013, the company said this in a Webmaster Central blog post: “Many websites embed videos in a way that works well on desktops but is unplayable on smartphone devices. For example, if content requires Adobe Flash, it won’t be playable on an iPhone or on Android versions 4.1 and higher.”

“Instead of using a proprietary video player or putting content in unsupported formats, we recommend using HTML5 standard tags to include videos or animations,” Google says. “For animated content rendered using Flash or other multimedia players, consider using HTML5 animations that work across all web browsers. Google Web Designer makes it easy to create these animations in HTML5.”

More on Google Web Designer here.

To learn more about Google’s new mobile-friendly signal and what you can do to get your site mobile-friendly, read this.

And for what it’s worth, Bing employs a similar philosophy.