ID vs CLASS in CSS. Please explain in detail with example

Question

I have heard that ID is unique and can only be used once in a page, but its working fine when used over multiple times on a page. Please let me know the purpose of ID and hows i

Answer 1

If you care for XHTML/HTML validation, then you should use unique IDs.

See this:

Class vs. ID

And

Why must the ID attribute be unique on each page?

Hope it helps.