GetDesignTimeHTML - This function returns a string of HTML,
This is used by VS.NET (or other host) to render the control at design-time.

GetEmptyDesignTimeHTML - This function returns no HTML or HTML
for an empty control (I will go over this in more depth later.

GetErrorDesignTimeHTML - This function returns the HTML to
render when there is an error in displaying one of the other two. The
default is the standard error message (which I'm sure you've seen).