Yeah, they usually do their own thing. Not only in following specs, though. Like how if you use self-signed cert for HTTPS (e.g. for local development and tests while playing with WebRTC), all browsers will show a Warning page with an escape hatch that can be clicked to ignore the problem. You can grab your personal phone, or ask anyone to lend you their phone in a pinch, and check your marvelous web creation.

Did I say "all browsers"? Sorry, not quite. iOS Safari outright refuses connecting. And it won't say why. I had to connect the damn iPhone with a cable to an Xcode machine, to open up browser console and see the error there. Turns out Safari is a special snowflake that requires doing some preparation ahead of time (no less than 3 steps!) 1) getting the Root CA into the phone, 2) install it in the cert storage, by the way you won't be able to do it if used a non-Apple app for step 1; and 3) enable full trust in the new CA. All that for a phone that I quickly borrowed from my flatmate. Thanks but no thanks! Not all developments are enterprise projects with proper laboratory devices.