@jackie_w - I didn't write that particular feature, it was done by Idolse. The discussion on it all starts
here. I've never touched it other than a couple of tweaks necessary as other bits of the plugin got refactored or if I hit issues (like I found recently of Sigil 0.5.3 "hiding" text that was inserted into a css file by this feature unless it got inserted at the top rather than the bottom).