I thought Safari’s release notes are pretty good – exhaustive, direct, something you won’t ever read for pleasure, but something you can actually learn a lot from even if you are just scanning.

I wish either MDN or Can I use… integrated them in some way (and, of course Chrome’s and Firefox’s as well), so that looking up a certain feature or property – for example, will-change – would show you the recent changes in browsers in reverse chronological order, which could help you understand the details of the feature evolving, and help with debugging.