No matter how popular a browser is, once the developer stops supporting it, eventually its usage will start to decline and even those who make web services available will find themselves unwilling to continue supporting it. Such is the case with Internet Explorer 6, which just a few years ago was the world's most popular browser and now is getting the cold shoulder from YouTube.
Users of IE6 are reporting that YouTube now displays a banner to them warning that the service will soon be phasing out support for the browser. The logos of Google Chrome, Firefox, and IE8 appear right next to the text encouraging users to upgrade to a more modern browser.
Security experts and even Microsoft itself have also been urging users to upgrade as fast as possible. So long as major sites continue to provide support for IE6, however, the longer it will take for it to disappear entirely - so when a major site like YouTube announces their intention to say goodbye, it can generally be seen as a good move. We can probably expect a more formal declaration soon.