Form navigation mental model

If an user comes from Windows based software, where in forms you navigate through the inputs with enter key and save by ctrl+s, to browser based software where the default navigation is done with the tab key and save with enter, what can I assume about user's mental model?

If it's a power user who has filled Windows based forms rapidly with only keyboard, and to put it extreme, with only number keys where you can type in a number, hit enter, type in a number, hit enter, et cetera.. should we assume the user expects same kind of behavior in the browser or should we assume he/she uses the browser software as he/she has learnt to use in other browser based forms (as in, navigate with tab and submit with enter)?

Power user functions, keyboard shortcuts etc are a great add-on to a browser software, but when it conflicts with the default way of the browser handling keyboard events, I think it is quite dangerous and too glitch-friendly code (with few exceptions such as word processing software like Google Docs.. but the main point is to think about form based software).

Thus I find myself in a dilemma and I need different views.