Likely YouTube Dailymotion also adds a play icon in browser tab to show video is playing. Not more than a week ago on the same blog we also broke the news about the addition of new unicode character on tabs that are playing video by YouTube.
This is quite useful feature when user has to pin point the noisy tab to close or pause the playing video among multiple opened pages. The icon will disappear as you finished or click media player to pause video.
Dailymotion is video website and counted as a YouTube alternative. This addition is seems to be small but obviously it will surely help to improve user experience.
Recommended: Create Twitter Video Profile