Question aboout ids and classes in smarty templates

@developers @helpers
Currently, I am working on the issue that the size limit for images to be uploaded is not shown in the upload dialogue. To display this information, I want to extend the photos_upload.tpl smarty template. I guess the best way is just to insert a new <div>...</div> beneath the submit button. But I see that any <div>...><div> has a unique id and class attribute.

What's the deal with these two attributes, what do I need to consider? Are they mandatory or optional?
And for this very specific case: When I insert <div>...><div> for the image size limit, which values should I use for id and class attributes?