HTML spec - when does form submission occur?

Hello,

Apologies if I've missed something obvious, but I can't find anything 
in Chapter 17 of the HTML 4.01 spec about the circumstances under 
which a form gets submitted. There's a description of what happens 
when the form is submitted, but not what causes it to be submitted. 
Maybe you consider this aspect of the browser's behaviour to be 
outside the scope of the specification, but I didn't find a statement 
to that effect either.

Specifically, I'm interested in what happens if you press <RETURN>, 
and whether's there's a default submit action that you can suppress. 
Some quick experiments with a couple of browsers suggest that you can 
submit a form without a submit button by pressing <RETURN> in a text 
item, but this behaviour doesn't seem to be documented.

Thanks,

Robert Stroud