Is there a way to comment out markup in an .ASPX page?

Bonus answer: The keyboard shortcut in Visual Studio for commenting out anything is Ctrl-KC . This works in a number of places, including C#, VB, Javascript, and aspx pages; it also works for SQL in SQL Management Studio.

You can either select the text to be commented out, or you can position your text inside a chunk to be commented out; for example, put your cursor inside the opening tag of a GridView, press Ctrl-KC, and the whole thing is commented out.

<%--
            Commented out HTML/CODE/Markup.  Anything with
            this block will not be parsed/handled by ASP.NET.

            <asp:Calendar runat="server"></asp:Calendar> 

            <%# Eval(“SomeProperty”) %>     
--%>

Source

FYI | ctrl + K, C is the comment shortcut in Visual Studio. ctrl + K, U uncomments.