I have specifically mentioned SRI - but matching the initial script out-of-band borders on the impossible (or at best "highly improbable"): you need two things, one is easy and one is hard. The easy part is verifying the hash of the initial page - a browser extension could do this (running JS again, oops); and the hard part is a trusted way of
obtaining the hash OOB. "Takes some knowhow" doesn't even
begin to describe the issues. Where are you obtaining this hash from, and how are you verifying that it's actually a legit and not a malicious one? (It's signed by the author's pubkey...which is verified how?) That doesn't "take some knowhow" - that takes a whole framework, half of which is currently imaginary.
You're handwaving that away as "oh, it's a simple matter of building it into the platform," where "it" is amongst other things a public key infrastructure and a secure software distribution system built with it. Easy peasy, right? (Spoiler: no) Contrast to a binary that's distributed through the platform's install/update system - all this is already built, and there's pretty good assurance that you're not getting a malicious result (signed packages).