Do checkbox inputs only post data if they're checked?

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 defa

Answer 1

From HTML 4 spec, which should be consistent across almost all browsers:

http://www.w3.org/TR/html401/interact/forms.html#checkbox

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.

Successful is defined as follows:

A successful control is "valid" for submission. Every successful control has its control name paired with its current value as part of the submitted form data set. A successful control must be defined within a FORM element and must have a control name.