If I have an HTML file and a CSS file both open in notepad++, only the HTML file can be previewed. If I edit the CSS file, rather than showing me the preview of the HTML file with the new CSS applied to it, your plugin says 'no preview available'. My request is that if the active tab has no preview available (ie is not an HTML file), display the last active HTML file in the preview. That way, when we edit CSS, we can still see the preview changing in real time.

Open your html and css in two separate instances. (Drag the second tab onto your desktop.) You can then preview the html and edit the css seeing any changes as you type them :)

*Couldn't edit my previous comment*
Actually you need to press refresh, but that's no big deal.