We created a little chat client in our office that runs in the browser. It handles people posting pictures and gifs and embeds them. Recently we added support for imgur and gfycat's html 5 video (it auto converts gifs to mp4/webm). An unintented side effect of this is now anyone using chrome or firefox will find that their monitor will never turn off.
You can see in powercfg the culprit is these videos:
C:\WINDOWS\system32>powercfg /requests
DISPLAY:
[PROCESS] \Device\HarddiskVolume2\Program Files (x86)\Google\Chrome\Application\chrome.exe
Playing video
[PROCESS] \Device\HarddiskVolume2\Program Files (x86)\Mozilla Firefox\firefox.exe
If you pause the videos, the entries in powercfg disappear. Switching to another tab, minimizing chrome, even locking the screen don't allow the system to turn the monitor off. Internet Explorer seems to be the only browser that behaves like you'd expect and only prevents sleep when a video is fullscreen.
Is there some way to mark a video tag as 'unimportant' and basically tell the browser that it's OK to let the computer sleep while this video is playing?
It seems there is a bug in Google Chrome 38 (current release as of this writing) that causes the screen saver to be suppressed when playing HTML5 video. Normally, screensaver suppression should only occur when the video is playing in full-screen mode.