I can't find a definitive answer to how to use the disabled property to disable form elements in javascript.

Some places say it's a simple boolean. Other's say to set it to the string value 'disabled' to disable, empty string '' to enable. Or there's adding and removing the disabled attribute, usually in combination with a string value.

So which is the proper way to do it cross browser? (no jQuery).
How did something as simple as an enable flag get so butchered?

-- edit --

Sorry, I should have mentioned I'm developing for IE8 (not by choice).