HTML and CSS Reference
There is an emerging class of tools for authoring and generating CSS
code that makes small modifications to the language and allows helpers
such as variables and selector nesting to be used. These changes remove
some of the repetition and document searching that typically come with
building and maintaining styles for a larger site.
To turn the written code back into code in a syntax, browsers understand
a conversion layer (preprocessor) is added as a compilation step via tools
on your local development machine or via server-side scripts.
the CSS rules from one selector into another (a mixin), selector nesting,
simple mathematical formulas, and variables that stand in for values
so that colors, sizes, and other values need to be typed only once. The
following is an example of both a variable definition and a nested rule
as written using Less: