What practical problem will i face if i use same #id more than one time on a page , other than validation error “ID is already defined”?

Question

First of all I alwyas use one #ID. just asking this question to know deep reason behind it.

Is it only a matter of W3C validation? or more than that

Answer 1

The result is undefined.

Apart from what the rest said about getElementById:

• using #foo in CSS may apply to all elements with id="foo", or to the first, or to the last, depending on how the browser is implemented.
• using page.html#foo in a URL may scroll to one of the elements, or to the other, or to neither.

edit: either way, you should teach your students to do things right even if doing it wrong doesn't seem to have any immediate bad consequences... ;)