[GrannyGrump]

You might have several CSS stylesheets, and you might not want to use every one on every html file. For instance, you might have a stylesheet named Main.css that you use for each file that is in the body of the book. Then you might have an additional special one named Aux.css that you want to use only for a single html file that contains your Index. Maybe you only have three html files that include images, so you might want another special stylesheet named Pics.css containing only stylings for your illustrations, so you could link that one only to those three html files. Etc, etc, etc.

Each (x)html file can be linked to multiple stylesheets.