The one issue with this approach (just creating the live doc when you switch to a CSS file in the editor) is that it won't pick up changes that might happen to the CSS file other than through the current editor - e.g. external changes from LESS/SASS recompilation or switching branches in git, or (eventually) from things like multi-file find/replace operations.

Ultimately, I think we either need to:

1. Mimic the original behavior, by having the browser side send us notifications for all the stylesheets it tries to load, or
2. On the Brackets side, detect whenever a CSS file (or other kind of file that could have a live document) is requested from one of our live dev servers by the browser, and create a live document at that point.