Believing you can do so and not fuck it up is sheer hubris, not to mention, wrong. Doing so because you don't like the browser edge cases, is how websites end up suffering from even more edge cases, and harks back to the bad old days of "Sorry your browser is not supported".
Financial institutions are amongst the worst offenders - ironic since pulling this kind of naïve stunt vastly increases the attack surface. I've been in a position to redesign financial institution systems and rewrite their technology policies, and when doing so this kind of off-standard NIH crap is something I seek to stamp out.
Want to submit a form? Use a damn form, with a plain old submit button, and rely on the standard behaviour. Want to put some UI polish around it? Progressively enhance the standard behaviour.
What not to do: imagine you can write better UI<>protocol interaction code than that already in Chromium and Webkit, or hope to infer and reimplement the behaviours of any more specialised user-agents.