The infrastructure to do signed OS loading is already in place, and on some operating systems (e.g. Android), the OS attestation service is already in place. So everything is mostly in place already to have your browser attest that it is official Google chrome on Google Android on an approved device with a hardware chip that verifies a Google approved boot signature. That hardware chip contains a Google approved private key (a key that's signed by a manufacturer that Google has in turn approved/signed) that can't be extracted, and that's the key that makes the attestation. Replace the hardware boot verify chip with one that will verify software you want, and you lose your attestation key.
They could also make the OS service reach out to a web service to get an attestation that the attestation key hasn't been revoked, so even if someone did physically extract the key from hardware and share it, it could be revoked (assuming each device gets its own key).
In effect, wide use of this kind of thing means that open source software is no longer free since even if you can look at the code, you must be part of the anointed class (i.e. working within our approved by a major corporation) to edit it and run your edits.
[0] An ARM exception level that sits above hypervisors and is specifically intended to support trusted execution modes for isolated mini-operating-systems that do this sort of shit
Because that thing basically describes a proprietary plugin like Activex, Silverlight or Flash before it, so a third party browser which doesn't have that proprietary tech can't fake it, under pretense of "standard". The code of that plugin will not be open source, worse, it will act as a spyware on people's computers at the OS level.
It's like EME before and these proprietary techs have no place in a open standard spec.