I have this

  #mydiv{font-weight: bold;}

<div id="mydiv">This is in bold</div>

If I wanted to, I can do this
<a href="#mydiv">This is in bold</a>

But how do get it to call the class, "#mydiv" when the page first loads up??
I tried
<body class="#mydiv">
<body onload="#mydiv">
but none of these code work. Any ideas??

I know you can just call a function in javascript but how do I call a class in CSS??