yeah, website code editing is not needed, blocking of resource loading and browser API editing is what is done, and of course attaching additional stylesheets. sometimes HTML "code" editing happens too, but that's probably not that important.

also browsers are called user agents for a reason. they should be an agent of the user, not of website owners, for the purpose of communicating with the website on the behalf of the user

You could argue the browser is NOT showing your code the way you intended (e.g. "Btw hello World" being rendered though I'm not sure if spaces would be there or not).

and that shouldn't be legally required either. for one the web standards were not developed as laws of a government, but there's also software bugs and unspecified behaviors, website owners should never be able to sue browser makers for not showing their website exactly as they expected.