Essentially, style sheets are written instructions explaining how a certain document should be displayed.
Style sheets are as old as the printing press and probably older.
In the days of manual type-setting, style sheets were nothing more than a set of written instructions from the publisher to the printer telling the printer what kind of style to use when printing up the publisher's manuscript. The printer would then consult the style sheet of that particular publishing house for a range of specifications. These specifications would encompass such details as what size the pages were to be, what size and family of font to use for chapter titles, sub-headings, body text, and so on. Plus, how much leading to put between the lines, what margins to leave, and whether, and by how much, to indent the paragraphs. As we have said above, HTML is a markup language that helps define how the web browser should display a given HTML marked up document.