In CSS, more than one style sheet can influence the presentation simultaneously. Both readers and authors can influence the presentation through style sheets. The UA is free to choose the mechanism for referencing personal style sheets. Sometimes conflicts will arise between the style sheets that influence the presentation.
Conflict resolution is based on each style rule having a weight.
By default, the weights of the reader's rules are less than the weights of rules in the author's documents. I.e., if there are conflicts between the style sheets of an incoming document and the reader's personal sheets, the author's rules will be used. Both reader and author rules override the UA's default values. The imported style sheets also cascade with each other, in the order they are imported, according to the cascading rules defined below. Any rules specified in the style sheet itself override rules in imported style sheets.