Tools → Options → Web Forms Designer → Start pages in “Source view”.
In VS < 2015 it was called HTML Designer.
For other extensions you can define specific behaviour by right clicking on a file and selecting Open With. This allows you to set once off or default behaviour for opening more esoteric file types.
For example, I use the Sandcastle Help File Builder, so have .shfbproj extensions loading up the SHFB GUI, but you could also load it up as plain XML (or whatever).