The HTML5 draft defines:
The value sanitization algorithm is as follows: If the value of the element is not a valid floating point number, then set it to the empty string instead.
http://www.w3.org/TR/html5/forms.html#number-state
I suppose you'd have use a default value of "0" to make sure wether the field was left untouched or if something invalid was entered, since there seems to be no obvious way to differentiate the two.
EDIT2:
After reading up validation specs and some testing (in Opera 10.54) I concluded that:
<input id="email" type="email" value="blah">
document.getElementById("email").validity.typeMismatch // True
Doesn't work on <input type="number">. Not sure if it's supposed to, or if it's a work in progress. The property does however exist, though it always returns False.
Read more: http://www.w3.org/TR/html5/forms.html#dom-validitystate-typemismatch
You can also set a custom validation method: http://www.w3.org/TR/html5/forms.html#dom-cva-setcustomvalidity