I´ve never used this feature before, so I will have to take a look at the specs first.
If its just an HTML attribute, it should be easy enough and should not require any scripting. Devtools is not great for testing this, because it can make a difference, if you add things later (DOM not rechecked afterwards).
Keepass will however not work without browser plugins, this was just asked yesterday: https://sourceforge.net/p/keepass/bugs/2269/
Which application do you use for autofill?