If you use Tabs control from AJAX Control Toolkit library you may have noticed that the default CSS definition provided with the control does not display properly when you increase font size on a page, that is the default CSS is not scalable.
Since Tabs allows you to override CSS it's quite easy to fix a problem. Just grab a correct CSS file with the set of images and assign a CssClass property a new class name:
<ajaxToolkit:TabContainer runat="server" CssClass="myTabs" ...>
Now it looks more appropriate: