Web page authors can now focus on using HTML to exclusively define the structure of a document with headers, paragraphs, lists, and bolding, while using CSS to control the format of presentation, including margin widths, white space, font weight, etc. Through CSS an author can specify a certain class of hyperlinks to be, for instance, uppercase, Arial font with a red underline. CSS rules can be embedded into the HEAD of an HTML document. That will potentially allow external stylesheets rules to be merged with existing stylesheet rules. Some web browsers support reader-stylesheets that let users specify a local CSS file to use for formatting all web pages viewed.
Some browsers also give users the option to turn stylesheets off.
The fact that both author and viewer can impose stylesheets in the same web page raises the question of who has control of the presentation. An author may insist that their page design is to be maintained at all instances while the reader asserts that they have the right to alter presentation to meet his or her needs, which can include increasing accessibility, legibility or usability. With the multiple ways to incorporate CSS rules into a web page there is a question of how much control is with the author and how much control is with the viewer. This is oddly and unfortunately is not the case with the current implementation of reader-stylesheets and Microsoft Internet Explorer 4.0. If the viewed web page has CSS rules then these are given precedence over the reader stylesheet.