Web Design in a Nutshell, 3rd Edition

disabled and readonly

The disabled and readonly attributes inhibit the user’s ability to select or change the form field. When a form element is disabled, it cannot be selected. Visual browsers may render the element as grayed-out. The disabled state can only be changed with a script. This is a useful attribute for restricting access to some form fields based on data entry earlier in the form.

The readonly attribute prevents the user from changing the value of the form field (although, it can still be selected). This enables developers to set values for controls contingent on other data entry using a script.