To me it looks like SGX for the web. Maybe it will introduce some neat and weird capabilities, but at the end of the day, it will be trivial to bypass at scale if it ever positions itself as being harmful to users.
Let's say example.com decides to require attestation from the {MS, Apple, Google} providers, and that they attest to only Chrome without extensions. You can't forge the attestation because cryptography. You can't fail to provide it (because they'll just refuse to send the bits). You can't use a "malicious" attestor because example.com won't trust it.
What's the trivial bypass I'm missing? How does a freely accessible standard impact the ability to bypass things in any way?