Can someone please tell me how to submit an HTML form when the return key is pressed and if there are no buttons in the form? The submit button is not there
Using the "autofocus" attribute works to give input focus to the button by default. In fact clicking on any control within the form also gives focus to the form, a requirement for the form to react to the RETURN. So, the "autofocus" does that for you in case the user never clicked on any other control within the form.
So, the "autofocus" makes the crucial difference if the user never clicked on any of the form controls before hitting RETURN.
But even then, there are still 2 conditions to be met for this to work without JS:
a) you have to specify a page to go to (if left empty it wont work). In my example it is hello.php
b) the control has to be visible. You could conceivably move it off the page to hide, but you cannot use display:none or visibility:hidden. What I did, was to use inline style to just move it off the page to the left by 200px. I made the height 0px so that it does not take up space. Because otherwise it can still disrupt other controls above and below. Or you could float the element too.