By default, HTML elements within an ASP.NET file are treated as literal text and you cannot reference them in server-side code. To make these elements programmatically accessible, you can indicate that an HTML element should be treated as a server control by adding the runat="server" attribute. You can also set the element's id attribute to give you way to programmatically reference the control. You then set attributes to declare property arguments and event bindings on server control instances.

Note:
HTML server controls must reside within a containing form tag with the runat="server" attribute.