Why do html checkboxes only transmit an 'on' value?

Question

I'm just starting to learn internet programming.

After playing around with checkboxes, I found out the hard way, that it is only when the checkbox is checked that it transmits an 'on' value.

Why not also send an 'off' value?

Answer 1

It is possible to have many checkboxes with the same element name:

<form action="order.php" method="get">

  <p>
    <label><input type="checkbox" name="toppings" value="olives"/>Olives</label>
    <label><input type="checkbox" name="toppings" value="mushrooms"/>Mushrooms</label>
    <label><input type="checkbox" name="toppings" value="onions"/>Onions</label>
    <label><input type="checkbox" name="toppings" value="eggplant"/>Eggplant</label>
    etc...
  </p>

  <p><input type="submit" value="Submit"/></p>

</form>

The url will later include only the checked values and will look something like:

order.php?toppings=mushrooms&toppings=onions&toppings=eggplant

This would have not worked well if an "off" value was sent as well.