HTML5 pages that expect to remain visible, or to otherwise prevent the system from entering a low-power mode after a long period of no user interaction (eg. YouTube or SoundCloud), are currently not able to distinguish themselves from pages that just use media elements for minor flair.
Chrome has an extension/app API for this in
chrome.power. I think this might be a solid base for a new specification (putting
navigator), with a proposed UX for pages that works like the “this tab is playing sound” icon (with icons for “this tab is keeping the [system/display] awake”). Navigating away from a page would automatically release that page’s keep-awake handle.