If they overcome / disable ad blocking, they will lose browser market share - and people don't design websites for marginal browsers with exotic features.

I find it hard to see how they could protect content from ad blockers without also crippling pages that self modify their own content. Perhaps they could put headers akin to content security policy that forbids external modification. Assuming a browser were to honour that header I could see bad publicity and a lot of people just moving to another browser which doesn't. Additionally, ad blockers aren't the only things that modify pages - breaking accessibility add ons could be more negative publicity (just like with Reddit).

I think browsers would be best off to let websites develop countermeasures if they're so sore about ad blockers. Perhaps they could use "self healing" Javascript libraries that put back content which is removed. Or they could just refuse to work if they detect an ad blocker, e.g. they stick some canaries in the DOM or along blocked paths to see if an ad blocker is present.

Having thought about it for a bit, it's possible for this proposal to be abused by authoritarian governments.

Suppose a government—say, Wadiya—mandated that all websites allowed on the Wadiyan Internet must ensure that visitors are using a list of verified browsers. This list is provided by the Wadiyan government, and includes: Wadiya On-Line, Wadiya Explorer, and WadiyaScape Navigator. All three of those browsers are developed in cooperation with the Wadiyan government.

Each of those browsers also happen to send a list of visited URLs to a Wadiyan government agency, and routinely scan the hard drive for material deemed "anti-social."

Because the attestations are cryptographically verified, citizens would not be able to fake the browser environment. They couldn't just download Firefox and install an extension to pretend to be Wadiya Explorer; they would actually have to install the spyware browser to be able to browse websites available on the Wadiyan Internet.

I reported him on github, for all the good that will do.

Non-goals [...] Enforce or interfere with browser functionality, including plugins and extensions. [...]

But guys they gave their pinky promise it's totally fine

let's just allow them to irreversibly make this change so that there is nothing preventing them from applying this totally Non-Goals in the future what could happen

Before everyone starts complaining, remember:
This is for the ads. There are millions of starving ads on the internet right now. For just a click and load a day on every ad you see you too can help a billion dollar company survive.

"Please click on all the crosswalks before you can enter this site"
This is why people pirate things.

This comment is hilarious: https://github.com/RupertBenWiser/Web-Environment-Integrity/issues/51

It doesn't seem to be targeting ad-blockers in particular (or other page customizing extensions), although that may result eventually. What it does do is let webpages restrict what web browsers and operating systems you are allowed to use, just like how SafetyNet on Android lets apps restrict you to using an OS signed by Google. That could end up with web pages forcing you to use a web browser and OS the big players like Google, Microsoft and Apple, blocking any less restrictive or less used competors like Firefox and Linux, thus creating a cryptographically enforced oligopoly. And even if they signed e.g. Firefox, it would only be certain builds of it. That would make it impossible to make a truly open-source browser that can access pages using this API. Quite concerning.