CSS won't work

[rotpar]

I have a css problem, placing a div into the ad_header... for exapmple

I place this code in the ad_header template:

<div id="mybanner">bannercodeblabla</div>

in my extra.css for example I write

.mybanner {float: right;}

The problem is, that the css is not applied to the div. What am I doing wrong?

 

[xf_phantom]

. = class selector
# = id selector
http://www.w3schools.com/cssref/css_selectors.asp

.mybanner {float: right;}