Is there some standardized markup for "instead of using the default username/password you have saved for this site, here's a string that will help you identify this set of fields so you can save the password separately if you really want to" or something similar
Yeah. Tell your browser to not remember your passwords for this site.
Sure, I'll just keep 63 case sensitive random letters and numbers memorized for each site.
That's what password managers are for. Else how did you get random letters and numbers you can't memorize?
You just told me to disable my password manager for this site.
A browser ought not be a password manager. What happens if you need to change computers? What happens if you need to wipe that one? Or you lose it or it gets stolen?
That's why you sign in to the browser, which (presumably) clouds your data for
them to sift through.
Yeah, good luck sifting through the data that Chrome encrypts client-side with a password other than my Google password, Google.