Re: For those wondering why?
"That seems reasonable; no one wants a Windows 10 update to start just as a battery is going flat, and, just as it would be nice if local apps could take account of battery condition, why not the same for cloudy-based apps?"
The approach taken is dumb for the following reasons:
1) Not all batteries are made equal, so a remote application has no idea how to interpret the battery information.
2) The remote application has no visibility over the user's usage patterns or other applications running locally and can't actually predict them as well as the user can.
3) It's likely that users will have no control or visibility over how a website reacts to the battery info, their web browsing session will change on the hoof without warning.
5) There is a really simple way to accomplish the same goal of power saving without changing anything: Have the websites provide a "low-power" version of themselves and let the user navigate to it using a bog standard link. If users care enough about low-power they'll find the link and even better they will be in control.
Because it's such a bad fit for the problem at hand I suspect that automatically pushing low-power webpages isn't the main goal behind this particular piece of crap.