Every browser seems to treat pressing Enter as submitting a form. Is this behavior specified by HTML standards, or is it just commonly implemented by browsers?
Related though goes into much more detail about the exact behavior of buttons: Do all browser's treat enter (key 13) the same inside form?
The closest I've found, from https://www.w3.org/TR/html52/sec-forms.html#implicit-submission:
If the user agent supports letting the user submit a form implicitly (for example, on some platforms hitting the "enter" key while a text field is focused implicitly submits the form), then doing so for a form whose default button has a defined activation behavior must cause the user agent to run synthetic click activation steps on that default button.
This suggests that it's platform-dependent.