Did you know that you can use HTML5 input types with ASP.NET WebForms controls? I wasn't sure until I tried it today:

<asp:TextBox runat="server" ID="Username" Width="250px"  type="email" />

which properly produces this HTML5 compliant HTML output:

<input type="email" style="width:250px;" id="Username" name="Username">

That this works shouldn't come as a big surprise since ASP.NET always has supported 'extra' attributes on Web Controls to render into the HTML. However, the input type seems like a pretty core feature in input controls and the control does have a TextMode property that already partially addresses this task. Nevertheless, explicitly specifying the type attribute allows you to override the type and so provide custom HTML 5 input types.

In short - nice! HTML5 input types are supported in WebForms without any framework changes or updates.

[updated from comments]
Apparently, in order for this to work you need an update for for the .NET 4 runtimes which was shipped with Visual Studio 2010 SP1. However on live servers you might have to install the Hotfix described in the previous link. I say might, because this code worked on my live server and I didn't explicitly install the hotfix - it appears that some of the .NET 4 updates through Windows Update must have addressed this. To be sure you should test a page that holds controls with HTML5 types to ensure the server will handle this - if not install the hotfix.