Different browsers may mark the input box in some way (Firefox 4 Beta adds a red box-shadow by default), display a warning (Opera) or even prevent the form from being submitted if this field has no value.
Hopefully these behaviours will converge in future releases.
Using CSS you can place markers inside or alongside the input box, or simply use background colours and borders as some browsers do by default. On the i Phone/i Pad the different input types are associated with different keyboards, making it easier for people to complete your online forms.
In other web browsers they can be used in combination with the .
A Java Script check is useful because it stops the form from being submitted if there is a problem, saving lots of time for your readers.