Author's style sheets override reader's style sheets which override the user agent's default values.
More specific declarations override more general ones.
This strategy gives author's style sheets considerably higher weight than those of the reader. A reader rule with an important declaration will override an author rule with a normal declaration. An author rule with an important declaration will override a reader rule with an important declaration. I'm told it never will because of absolute positioning problems. I agree, this would then depend on the browser.