can mootools change the attribute of a css .class so that the change is reflected across all elements that belong to that class? -

Similar to ...

I know I can Change the style of each element individually, but I want to change many element styles at the same time and the browser wants to fight around 40 elements.

thanks :)

Yes, it can be very easy.

  $$ ('. SomeClass'). AddClass ('NewClass');  

This will add newClass to each someClass element - in your case you have 40 elements.

This is the literal answer to your question.

That said, I believe that what you are actually trying to do is generate a specific CSS class on flying.

And it's far away, more complex. I suggest using this MooTools plugin: