number (1.5) and a unit (em). The "em" value in CSS refers to the font size of the surrounding text. Common style sheet languages have around ten different units. Value propagation mechanism To avoid having to specify explicitly all values for all properties on all elements, style sheet languages have mechanisms to propagate values automatically. The main benefit of value propagation is less-verbose style sheets. In the example above, only the font size is specified; other values will be found through value propagation mechanisms. Inheritance, initial values and cascading are examples of value propagation mechanisms. Formatting model All style sheet languages support some kind of formatting model. Most style sheet languages have a visual formatting model that describes, in some detail, how text and other content is laid out in the final presentation. For example, the CSS formatting model specifies that block-level elements (of which "h1" is an example) extend to fill the width of the parent element. Some style sheet languages also have an aural formatting model. References