I have implemented the styles to asp controls so we have to choose the property CssClass="TestStyle but in case of html controls it works like Class="TestStyle"
<%@ Page Language="VB" AutoEventWireup="false" CodeFile="Default3.aspx.vb" Inherits="Default3" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<style>
.TestStyle
{
padding: 0.25em;
background: #adc175;
float: left;
margin: 1.5em;
font: bold 105% times new roman, serif;
border: #82a91b 2px solid;
width: 10em;
}
</style>
</head>
<body>
<form id="Form1" runat="server">
<asp:DropDownList ID="DropDownList1" runat="server" CssClass="TestStyle"
Height="16px">
<asp:ListItem>a
<asp:ListItem>b
<asp:ListItem>c
<asp:ListItem>d
<asp:ListItem>e
<asp:ListItem>f
<asp:CheckBox
ID="CheckBox2" runat="server" CssClass="TestStyle" Text="click" />
<select id="Select1" class="TestStyle">
<option>a</option>
<option>b</option>
<option>c</option>
</select>
</form>
</body>
</html>