I would expect this:
Web controls in the System.Web.UI.WebControls
namespace may render differently in different browsers. You can't count on them rendering the same elements always. They may add anything that they think is needed to make it work in the specific browser.
If you want to have any control over how the controls are rendered as html, you should use the controls in the System.Web.UI.HtmlControls
namespace instead. That is:
<input type="checkbox" id="CheckBox1" runat="server" class="myClass" />
<input type="radio" name="RadioButton1" runat="server" class="myClass" />
<input type="text" id="TextBox1" runat="server" class="myClass" />
They will render just as the corresponding html element, with no extra elements added. This of course means that you will have to take responsibility for the browser compatibility, as the control doesn't. Also, those controls doesn't have all the features of the controls in the WebControls
namespace.