Keys for <default> and cancel should be Enter and Escape and nothing else
Currently the default keys on dialog boxes vary.
Mpt reported this bug in bugzilla in 2005 here is the suggested solution from that bug. As far as I can tell, the recommendation still stands:
In any alert that has only one button, that button should be labelled
'OK'. Enter (and also Ctrl+Enter) should activate this button. Do not
also give it an access key, as that would slow people down.
In any alert or dialog that has two or more buttons, Escape should
activate the "Cancel" button. Enter should activate the main action
button, unless a multi-line text field is focused (and Ctrl+Enter should
activate the button regardless of focus). Do not also give either of
these buttons access keys, as that would slow people down. (These
guidelines apply only to alerts and dialogs, not to instant-apply
As special protection, Enter should not activate the main action button
if the resulting action is highly destructive (for example, erasing a
disk). In these cases do not make Enter do anything else either, as
people may press it expecting it to work.
In a progress window, Escape should activate the only button in the
window ("Cancel" or "Stop"). Do not also give it an access key, as that
would slow people down.