I discovered something surprising:
<html>
<head>
<script type="text/javascript">
function f()
{
document.getElementById("h").value++;
document.getElementById("x").value++;
}
</script>
</head>
<body>
<form>
<input type="hidden" name="hidden" id="h" value="5"/>
<input type="text" id="x" value="5"/>
<input name='clear' type='reset' id='clear' value='Clear'>
</form>
<button type="button" onclick="f()">Increment</button>
<button type="button" onclick="alert(document.getElementById('h').value)">Show hidden</button>
</body>
</html>
Trying this in Firefox 4.0.1, clicking clear always resets the text input to 5, but never resets the hidden field.
I (and others) did not expect this behavior at all: we expected the hidden value to get reset too!
Can anyone point to either documentation or specs that explain why the hidden input is treated differently by the reset button?
Explanations as to why such behavior is desirable are also welcome.
FWIW, I think I can put together the full story from the answers and comments.
Usage rationale: The clear button is intended for clearing user input, and since hidden inputs are not directly accessible by the user, it doesn't make sense to allow the user to reset the hidden input's value.
Documentation and behavior:
The bug report that AR pointed out is explicit about what is happening: The hidden field's value's mode is default, as is intended in the specs.
Particularly, this means that changing the value (as in the sample code in the question) changes the default value, and the reset button resets input fields to the default value, hence there is no change.
The behavior for the text input is different (even though its value is also changed programmatically) because its value's mode is not default but value, which means that there is a distinction between the default value of the input and the current value.