HTML buttons have a form attribute that can be used with buttons of type="submit".
The form attribute is used to target a specific form.
By default a button of type submit will submit the form that it is a child of.
In some cases it may be necessary to submit a form that the button is not a child
of. This is where the form attribute comes in to use.
Example usage
Let's say we have a confirmation modal that appears when a button is clicked. This could be to confirm whether or not a user wants to commit changes to a resource. It is likely that the HTML for the modal dialogue is not contained within the original form. So when the confirm button is clicked inside the modal it will not submit the original form. Unless of course JavaScript is used to listen for the confirm event.
Adding a form attribute to the confirm button specifying which form to submit is a
much nicer way to solve this. For example the button below will submit the form
with an id="updateResource". Without requiring any extra JavaScript.
<button form="updateResource" type="submit">Confirm</button>