1. Open HTML page
2. Open HTML Preview. Preview is showing previously opened HTML page
3. Open CSS file linked to opened HTML page.
4. HTML preview still should show preview of opened HTML page not preview of opened CSS file (which is empty). User wants to make changes to CSS file but to see visual results of changes he made he must activate editor with opened HTML page. Would be more useful to see results of changes made to CSS file immediately after CSS file is saved.
The same applies for JavaScript files or other files which are included to HTML file (templates...)