What being said from the first reply is true. Id should be unique in per page, i.e can only be use once. Where as class can be use repetitively.

Some might not see the importance of this, as they tend to use id instead of class again and again.

As for me, I followed this rule strictly since I'm using jQuery, a javascript framework for my works. On events such as, click, submit, etc, jQuery heavily depending on id and classes.

I believe for those who work a lot with Javascript, to say the least, would see the importance in differentiating id and class.

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""If you must break the law, do it to seize power: in all other cases observe it." - Julius Caesar