Style sheets from these three origins will overlap in scope, and they interact according to the cascade.
The CSS cascade assigns a weight to each style rule. When several rules apply, the one with the greatest weight takes precedence.
By default, rules in author style sheets have more weight than rules in user style sheets. Precedence is reversed, however, for "!important" rules.
All rules user and author rules have more weight than rules in the UA's default style sheet.