Imagine spending 5+ years in prison over copyright. I've seen news about violent criminals doing less time for violent crimes with actual victims.

The thing is he isn't circumventing copyright protection. He is just allowing it to play in his browser too, no downloading etc. I'm a legal noob, but there always was that "fair interests" like making stuff work on different devices/OSs (youtube-dl is hiding behind this a bit), would it not apply here?

“Legal noob” is definitely one way to say “I am absolutely not a lawyer”.

The ambiguity here is why I said potentially. Does circumventing a technical measure mean to access the content without Widevine, or is using Widevine in an unintended way enough? However, the law quite clearly says that fair use doesn't matter; you can't break DRM even if you're not violating copyright.

Would it be legal if DRM is not shipped with the browser, but the user can download and install it separately (copy it manually to the browser folder)?

Let's rephrase: can you create a clone, a derivative work of a working OS browser, which only has DRM part not changed?

I'm not sure if you're asking for technical or legal advice. If legal, then since most of us are not lawyers here, strictly speaking we can't judge if "Hello, world!" program will put us in jail or not.

Probably not, at the very least since Widevine is licensed by Google.