Bad practice multiple elements with same ID?

[Jezternz]

Okay,
Basicly I want to style multiple input fields with a new border, background ect.
However when I use a class they do not change in FF, when I give them all the same ID they do change in FF. I am building a web application, however all elements are accessed through javascript references and I do not access anything via getElementById, so from my point I do not need them to have different ID's.
But I was wondering if it breaks or damages the DOM at all? is it bad practice? and does this make it invalid xhtml?

Thanks, Josh

[Dormilich]

But I was wondering if it breaks or damages the DOM at all? is it bad practice? and does this make it invalid xhtml?

No, Yes, Yes.

well, it's what an ID is about, an identifyer. ( I never had problem through styling with classes…)

[Markus]

Maybe you can show us how you're styling the classes?

[Jezternz]

Have started working on something else, but will post my code when I get a chance.