This shouldn't be done at the HTML5 level, but at the codec level.
HTTP Live Streaming already provides a perfectly serviceable way to do exactly this: provide multiple streams at different bitrates and let the media library choose which is best for the network and output resolution.
This is probably one reason why Apple has been using its muscle to force all iOS apps to use HTTP Live Streaming. They knew about exactly this problem: iPhone app developers assuming you would never play the video on a large screen, but with AirPlay you will do exactly that.
HTTP Live Streaming already provides a perfectly serviceable way to do exactly this: provide multiple streams at different bitrates and let the media library choose which is best for the network and output resolution.
HLS doesn't solve this problem. HLS adjust bitrates, the problem here is resolution switching (which sometimes is involved in bitrate switching). On an iPhone the largest HD resolution you can fit on the screen is 960x540. On an iPad or OTT device (Roku, AppleTV) its 1920x1080.
Slight niggle, this shouldn't be addressed at the codec level but at the container/wrapper level.
HTTP Live Streaming already provides a perfectly serviceable way to do exactly this: provide multiple streams at different bitrates and let the media library choose which is best for the network and output resolution.
This is probably one reason why Apple has been using its muscle to force all iOS apps to use HTTP Live Streaming. They knew about exactly this problem: iPhone app developers assuming you would never play the video on a large screen, but with AirPlay you will do exactly that.