What part of the HTML specification prevents 120 Hz iPad Pro from doing 120fps browser animations?

I just ran MrDoob's FPS counter on Safari in a new Apple's iPad Pro (120Hz) but it only runs at 60fps, whereas a Chrome browser in a desktop monitor (with a refresh rate of 144Hz) runs up to 144fps.

The script uses requestAnimationFrame to calculate them, so it's not technically a resource-intensive consumption.

I couldn't find it on the WHATWG specification, but seems it should. References:

1. IE https://connect.microsoft.com/IE/feedback/details/794072/internet-explorer-animations-fails-on-120hz-computer-monitors-works-at-60hz-75hz-100hz
2. http://www.testufo.com/browser.html
3. Related: https://github.com/w3c/html/issues/785