I was just using the built-in spell checker in Chrome. After a little reading up, it looks like code mirror prevents the built-in spell checker from working.
Someone recommended integrating the typo.js plugin.
Stackoverflow Typo.js Integration Example
On the codemirror homepage, they recommend this plugin for ZenCoding.
GitHub Example/Repo
I tried to get it integrated, but didn’t have any luck. Also, I created a little patch to enable typekit, so I could control the font settings and pull in a bunch of sans-mono fonts. It might be cool to do the same thing but with google-fonts api.
