If possible you should have automatic tests that you can run automatically in several browsers.
Some browser need to be the first one to implement new functionality, before there is a specification! Then the specification comes and often it's different then the already implemented API.
You should not complain about new features, and you should not complain when browsers follow specifications. Instead you should have tests, and encourage users to report problems.
Most browsers have a beta/review release, you can run your test on that, once per day, automatically. Use a program that automate mouse movements and clicks, for example Puppeteer for automating Chrome. (tip: also do performance test while you are at it, so you will know which changes affect performance)