Hi there. I’m just getting my feet wet on using CodeMirror on our site and I really like it! I had one thing I ran into while learning how to configure CodeMirror that I wanted to mention, hopefully this is the right place for it.
I was struggling to get the HTML Mixed mode working and spent a while scouring the manual and examples for direction. After reading this Stack Overflow post and following the XML/HTML mode demo link in the answer, it redirected back to the CodeMirror site showing the docs for XML mode. When looking on the left side of the page, a Language modes link appeared that let me look at all the language modes supported. After checking out the HTML mixed-mode information, I found I wasn’t setting up several dependencies (Javascript, CSS, XML) that it relied on. After that, I was all set.
I thought it might be helpful to list the Language modes link in the manual explicitly on the left sidebar and/or maybe in the mode configuration description. I see now that the Language modes link was right at the top of the main CodeMirror page, but I totally missed it and was busy looking in the manual for it. 
Just a suggestion. Thanks for the great tool!