I don't always keep the same view over time - my view of things tends to evolve with the technology, so bear that in mind when quoting my posts from a few years ago. Also BTW it's kind of annoying to see people asserting that they know what I'm going to say, especially when they're wrong
tl;dr: star this Chrome issue: http://crbug.com/522983
My thoughts on this are:
Performance: I'm not convinced it makes sense to do this because the game might not reach 60 FPS. Modern mobile devices are about as powerful as laptops. Having one master switch to drop to 30 FPS on all devices is overkill IMO. I stand by this aspect of my previous post: if you are doing this to improve the user experience, then dropping a device that could easily reach 60 FPS to 30 FPS actually degrades the experience.
Also it's not just about mobile, some laptops have serious fillrate performance issues because they ship with the terrible combination of a weak integrated Intel GPU and a high-resolution screen. I think the original motivation for this feature was actually for these laptops. Intel integrated GPUs are far weaker than most mobile devices! (Before anyone accuses C2 of being slow, it's a hardware limit, native apps will be the same!) So if your intent is to flip this mode on for mobile devices and leave it off everywhere else, I think it might actually be better to leave it off for mobile and optionally on for underpowered desktop systems
Battery life: I think the argument here is better, although I would be interested to see actual data on the battery life savings. As far as I'm aware the screen is one of the biggest battery drain on mobile devices, so if the screen is on for the same amount of time at the same brightness, that part of the battery drain is identical regardless of how intensively the game is running. Also power-saving hardware works best when it can remain idle for long periods of time, allowing the chip time to fall back to the lowest-power modes. 30 FPS allows a maximum of about 30ms idle time, which isn't that much time to power down components. So I think it would be interesting to try and quantify this saving, rather than speculating that it will help. (Note people often compare 2D games to 3D games, but they're not the same - particularly in terms of overdraw, 2D games can be more intensive, since 3D games use different rendering techniques like Z buffering. Also modern GPUs are so well-optimised for 3D that it doesn't matter if you draw a triangle in 2D or 3D, it probably treats both as the same case.)
Also note that on a weak laptop plugged in to mains, there's no need to save battery life.
Browser support: we actually tried implementing this already, and it didn't work well at all, mainly because browsers are designed to run at v-sync rate. We tried simply skipping alternate frames, but the browser scheduler assumes a full frame will be drawn on the skipped frame. In some browsers this actually resulted in an even more janky mess, and in others it would incorrectly drop to quarter-framerate mode (just 15 FPS!) if the browser thought it couldn't hit 60 FPS, because the browser would drop down the framerate itself in some way. So there needs to be built-in browser support for half-framerate mode, which doesn't exist yet.
Based on that, I already made the case for this feature to some of the Chrome developers (making some of the same arguments people are giving here!), and you can star this bug to vote on the suggestion. Until then, there's not much we can do to actually support this.