I have been using elance contracts to design the more complex css elements of my website (the simple stuff I can do) and am a bit perplexed by one question.

Every contractor uses something like this:
<ul id="navbar">
And not something like this:
<ul class="navbar">

I thought that an "id=" cannot be attached to the same css rule more than once on the same page due to stability issues. For example, you could only use id="navbar" once on the same page, but could use class="navbar" repeatedly on the same page.

Thus, I have been switching the id="navbar" to class="navbar". Both seem to work and using class="navbar" means I can use a css rule repeatedly on the same page and not have to create two css rules that are the same but with different names to use them on the same page - reducing my css file size.

But the fact the elance contractors, who know more about css than me, are all using "id=" causes me to question is they are doing so for a good reason. Does anybody know the anwser?

Thank you!!!!!!!!!!!!!!