Help with CSS and Div Runat server

I have a <div> tag on my page that uses a CSS ID class to position itself and sets some text and background color attributes.  

Here's my problem.  If I load the page without the "runat" it loads with the correct CSS properties.  However, if I add the runat="server" to the div tag the CSS properties do not get applied to the div tag?

I need to add the runat because I want to control the divs visibility from the code behind.

So basically, this loads the CSS properties....

<div id="testPop">This is my div!.</div>

And this doesn't....

<div id="testPop" runat="server">This is my div!.</div>

What am I missing?  The page uses a masterpage so could that be the problem.  The CSS file is called in the masterpage.
     padding: 20px;

Open in new window

Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

use style instead:

<div id="testPop" style="styleName"></div>

<div id="testPop" style="testPop"></div>

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
cdemott33Author Commented:
It worked!  I'm not sure why but it worked?  What's the difference between the ID and the STYLE?
Determine the Perfect Price for Your IT Services

Do you wonder if your IT business is truly profitable or if you should raise your prices? Learn how to calculate your overhead burden with our free interactive tool and use it to determine the right price for your IT services. Download your free eBook now!

Here's what you need.  Just change the type of jquery animation to whichever kind you want.
What's the difference between the ID and the STYLE?

The ID can only be used on one element/tag.
The style can be used on many elements.
cdemott33Author Commented:
MikeMSCD - Understood but I'm still confussed.  Why can't I use an ID selectors?  Why must I use a style selector?  In my case (in my head anyway) it should not matter.  Ok, yes, I can only use it on one element, but what if that's all I want to use it for... one element.

I'm just trying to get my head around this one, because to me, it shouldn't matter.
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today

From novice to tech pro — start learning today.