When doing web development (or really any development on a complex enough platform), sometimes the best course of action for understanding puzzling behavior is to read the source. It's therefore been fortunate that Chrome/Blink, WebKit (though not Safari) and Firefox/Gecko are all open source (and often the source is easily searchable too).
One of exceptions has been mobile WebKit when accessed via a
Besides browsing around the source tree, it's also possible to track its development more closely, via an RSS feed of commits. However, there are no guarantees that just because something is available in the trunk repository that it will also be available on the (presumed) branch that iOS 8 is being worked on. For example,
Update on July 21, 2014: The selection granularity API has shown up in beta 4, which was released today.