HTML5 custom attributes - Why would I use them?

Question

I can\'t seem to understand why I should be happy with HTML5 allowing custom attributes? Why would I use them?

Answer 1

Custom attributes are already widely used, for example here's an example from dojoToolkit():

    

        

            Lorem ipsum and all around...
        
        

            Lorem ipsum and all around - second...
        
        

            Lorem ipsum and all around - last...
        
    

This could now be re-written so that the markup validates using attributes like data-dojoType. They also allow you to store application specific data in your tags rather than hacking around in the class attribute.

There's a good introduction to data-* attributes on HTML5 Doctor.