CSS - Cascading Style Sheets are a software option used by some of the web-design tools.
 
Style sheet enables the designer to define the text appearance, font style, paragraph, font size, italics or bold, colour, letter and line spacing, indents etc. It can be defined for a certain paragraph, page or the whole web base.
 
If you wish to change the font, you can simply change the settings of a style sheet and the change will appear in all the text related to that sheet. It enables the change of font in the whole base with a single mouse click.

Other than that, style sheets enable the designer to better control the design, additional code describing the variations in the design of the text on the page is not necessary, and pages are downloaded a lot faster.
But today different browsers support different specifications of style sheets which can result in unpredictable variations of the displayed text.