I just realized that there's an even more important case where this implementation won't work: when the user is editing styles using CSS Quick Edit. If you're in an HTML file, and set the cursor in (say) `<body>`, then hit Cmd-E to edit its style rule and modify it, you won't pick up the changes (at least if you haven't visited that file yet) because we haven't explicitly switched to that document before.

This case is shown in our smoke test - you might want to consider running that as you test the CSS live preview workflow: https://github.com/adobe/brackets/wiki/Brackets-Smoke-Tests

So I really think the right thing to do is to detect when a CSS file is requested from the browser and create a live document for it immediately at that point. LiveDevelopment should track those files using something similar to the "related docs" logic we had in the original version.