I mean that a reader might allow the user to specify some custom CSS, but if you add "!important" to the book's CSS, that will override the custom CSS, thus "breaking" the reader's feature.
I don't know if there are any dedicated reader that allows this (it's certainly something I'd wish), but I believe the Calibre viewer does. Say, for instance, that I like my books justified, so I put in my custom CSS:
Code:
p {text-align: justify;}
and all is fine. Then comes an author that thought that his book looks better left aligned and added this to his book:
Code:
p {text-align: left;}
No problem for me, my custom CSS has higher priority, and I see the text justified anyway. Then comes another author who only tested the book in Stanza, and added (assuming Stanza needs this, which I don't know):
Code:
p {text-align: left !important;}
which is not overridden by my custom CSS, because that's what "!important" means, and I'm forced to see the text left-aligned.