grey out submit button until form filled out

Question

I have the following form:

Answer 1

Jquery would do that. On the .keyup event have jquery check if both field's lengths are > 0 and change the button to be disabled or not.

$('#yourButton').button("disable");​​​​​​​​​​​​​

$('.fields').bind('keyup', function() { 
var nameLength = $("#sub_first_name").length;
var emailLength = $("#sub_email").length;

if (nameLength > 0 && emailLength > 0)
{
   $('#yourButton').button("enable");​​​​​​​​​​​​​
}

} );