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

If checkbox isn't checked then it doesn't contribute to the data sent on form submission.

HTML5 section 4.10.22.4 Constructing the form data set describes the way form data is constructed:

If any of the following conditions are met, then skip these substeps for this element: [...]

The field element is an input element whose type attribute is in the Checkbox state and whose checkedness is false.

and then the default valued on is specified if value is missing:

Otherwise, if the field element is an input element whose type attribute is in the Checkbox state or the Radio Button state, then run these further nested substeps:

If the field element has a value attribute specified, then let value be the value of that attribute; otherwise, let value be the string "on".

Thus unchecked checkboxes are skipped during form data construction.

Similar behavior is required under HTML4. It's reasonable to expect this behavior from all compliant browsers.