Question

Is it standard behaviour for browsers to only send the checkbox input value data if it is checked upon form submission?

And if no value data is supplied, is the default value always “on”?

Assuming the above is correct, is this consistent behaviour across all browsers?

Answer 1

Yes, standard behaviour is the value is only sent if the checkbox is checked. This typically means you need to have a way of remembering what checkboxes you are expecting on the server side since not all the data comes back from the form.

The default value is always “on”, this should be consistent across browsers.

This is covered in the W3C HTML 4 recommendation:

Checkboxes (and radio buttons) are on/off switches that may be toggled
by the user. A switch is “on” when the control element’s checked
attribute is set. When a form is submitted, only “on” checkbox
controls can become successful.