Saving passwords

In most places I visit with a password, Omniweb stores a Web form  
password on the Keychain.

But at least one place I regularly visit asks for a username and  
password which OW doesn't store, and of course I then have to retype  
the password everytime (or rather, open the Keychain, reveal the  
password, copy it to the Clipboard, and paste it into the form).

So: how does Omniweb decide that a form is the sort that contains a  
password, and hence will be saved and restored in the Keychain?

(This is a question which probably has an embarassingly easy answer.)