Visual Studio provides three different view modes for .aspx, .ascx and .master files: Design, Split and Source. Design shows a near-WYSIWYG interface, Source shows the HTML markup and Split shows a combined view with the source on top and the design below it. I've found a corner case that can stop the design view (standalone or as part of the split view) rendering inside Visual Studio but that doesn't seem to affect its operation at runtime.
Take a standard head tag configured to be a server control, as shown below. This works normally as you'd expect. But if you're excessively pedantic the title tag may irritate you because a closing tag isn't necessary.
<head runat="server">
<title/></title>
</head>
So you change it to use an empty element tag as below.
<head runat="server">
<title/>
</head>
Everything seems fine. Pages render, titles get set. All is good with the world. And then you switch to the design or split views and they don't render inside Visual Studio. This is somewhat irritating, because I did it on a master page which meant I had no design view for any page.
Obviously the fix is simple, but if it's been a while since you made the change (or someone else did it) it may not be immediately apparent why its happening. So todays lesson is not to be so pedantic about the trivial stuff. It's just HTML, it's not like we're arguing about where best to place a curly bracket.