If the user agent supports letting the user submit a form implicitly (for example, on some platforms hitting the "enter" key while a text control is focused implicitly submits the form), then doing so for a form, whose default button has activation behavior and is not disabled, must cause the user agent to fire a click event at that default button.
This technique is Sufficient to meet 3.2.2: On Input when used with G80: Providing a submit button to initiate a change of context.
It depends on what type of field it is in the form.
If 'enter' has a meaning within that form element, then you should have it behave according to that meaning. For example, if it's a textbox, 'enter' should take them to the next line. If it is a table, then 'enter' should move down to the cell below the current one.
If 'enter' has no meaning within that form element, it should submit the form.
The standard which you should us, it to 'tab' between fields, unless tab has a meaning within the form field. For example, if it's a table, 'tab' should move to the next cell.
— JohnGB, Mar 20, 2013 at 11:42