You settle in to watch a video, click play on YouTube in Safari — and instead of the video, a black box appears with the message: “your browser does not recognize any of the known formats.” The page itself loads fine and the comments are all there, but no video will play. If that sounds familiar, the culprit is almost certainly hiding in your extensions list, not on YouTube’s side.
Why Safari shows this error
With macOS Sierra and Safari 10, Apple made HTML5 the default video engine and stopped relying on Adobe Flash for web video. YouTube had already moved to HTML5 on its own, so everything should have become simpler — but many long-time Mac users had extensions installed whose whole job was forcing Flash playback, such as ClickToPlugin or ClickToFlash. Those extensions were built for the old Flash era: they intercept video pages and rewrite them to use the Flash player. With Flash effectively retired from Safari, that interception now breaks the page instead of improving it, leaving YouTube unable to find any playable format and showing the error you see.
Remove the Flash-forcing extensions
- Open Safari’s extension settings. In the menu bar, choose Safari → Settings (or Preferences on older macOS versions), then click the Extensions tab. Apple Support documents how to manage and uninstall Safari extensions if you want the official walkthrough.
- Look for Flash-forcing extensions. Scan the list for anything that mentions Flash playback, forcing video into Flash, or names like ClickToPlugin and ClickToFlash. 9to5Mac covered Safari 10’s switch to HTML5-first video when it shipped, and these legacy helpers were the main casualties of that change.
- Uninstall each one. Select the extension in the sidebar and click Uninstall, confirming the removal. Repeat this for every extension that forces Flash playback.
- Reload YouTube. Head back to the video and reload the page. With the interfering extensions gone, Safari hands the page its normal HTML5 player and the video plays.
If it still doesn’t work
After removing the extensions, most people are back to normal immediately. If a video still refuses to play, delete YouTube’s stored website data: open Safari → Settings → Privacy → Manage Website Data, search for youtube.com, and remove it. A content blocker can occasionally interfere as well, so try pausing it for YouTube temporarily. And while you are tidying up the browser, you may also like to change Safari’s design on iPhone or stop Safari from suggesting passwords for websites on iPhone.
FAQ
Why did this start happening right after I updated macOS?
The macOS Sierra update shipped Safari 10, which made HTML5 video the default and stopped loading the legacy Flash web plugin. Extensions designed for the Flash era suddenly started breaking video pages instead of improving them, which is exactly when this error appears.
Will installing Adobe Flash fix it?
No. Safari 10 ignores the Flash plugin even when it is installed, and Adobe has discontinued Flash entirely. Reinstalling Flash will not help — removing the conflicting extensions is the only fix you need.
Wrapping up
The “browser doesn’t recognize any formats” error in Safari looks alarming, but it is really a leftover from the Flash era: old extensions like ClickToPlugin and ClickToFlash fighting a browser that has moved on to HTML5. Uninstall them from Safari’s Extensions tab and YouTube plays normally again — no reinstallation, no workarounds, and no Flash required.
