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.