The !important rule is a way to make your CSS cascade but also have the rules you feel are most crucial always be applied. A rule that has the !important property will always be applied no matter where that rule appears in the CSS document. So if you wanted to make sure that a property always applied, you would add the !important property to the tag.
However, the !important rule was also put in place to help Web page
users cope with style sheets that might make pages difficult for them
to use or read. Typically, if a user defines a style sheet to view Web
pages with, that style sheet will be over-ruled by the Web page author's
style sheet. But if the user marks a style as !important, that style
will overrule the Web page author's style sheet, even if the author
marks their rule as !important.
This is a change from CSS1 to CSS2. In CSS1, author !important
rules took precedence over user !important rules. CSS2 changed this to
make the user's style sheet have precedence.