Why does ASP.NET still require runat="server" everywhere?

Internet Explorer supports DHTML behaviors. does not mean server control. You can create client DHTML component that has namespace and will run on the client machine. Also, namespaces are allowed in XHTML and techically you can use asp namespace for something else on a client, if you wish. Runat="server" prevents namespace clash. If element has no runat="server" attribute, it will be sent to the client browser unchanged. Therefore, you can use HTML components (HTCs) in ASP.NET pages as well

Use the runat=server attribute when you're using ASP.NET controls, and/or you require programmatic access to those controls in your code-behind.

HTML controls don't require this attribute. It's useful if you have any HTML element like <span> <div>, or <table> when you want the ability to access them in code-behind.

<asp:Label runat="server" id="foo" />
<div runat="server" id="bar />
...
foo.Text = "Hello Label";
foo.Attributes["class"] = "baz";