Why tab font size in not changing in runtime?

I'm using ajax tab control and trying to increase the tab font size but not working in runtime

Please suggest wehere i'm doing wrong?

 .tab         {             margin-left :100px;            font-size :large;        }

<div class="tab">                                         <ajax:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0"                         Width="1119px" Height="422px">                        <ajax:TabPanel runat="server" HeaderText="TabPanel1" ID="TabPanel1">                            <HeaderTemplate>                                Home                            </HeaderTemplate>