Through this website you can convert the font size you are using for website into EMs, percent and points.Also tThere is a standard CSS snippet given under the tab "Grab CSS". The inter relation of EM, Pixel, Percent and Point is there in the "Learn" section.
Very useful tutorial since the sample updates live while you're adjust your gradient.
In this Tutorial there is a mixture of checked pseudo-class and sibling combinators.
Tutorials and plugins.
In this article read the information about "LESS"(The dynamic stylesheet language).