The "!important" property is quite a little attention-seeker - it says, "look at me, I am more important than anyone else!". In this case, the code:
Code:
* {text-align: justify !important}
says "ignore other text-align properties anywhere else, do as I say instead!"
You should be careful when using "!important"; if you put it into an epub to be read by other people, then they may not thank you for it as you just made it more difficult for them to change the text-alignment to their own preference.
Conversely, it may be the only way for you to override the text-alignment in an epub that you want to read.
In summary, I would only use "!important" if it does not work without it.