CSS Pseudo-Class Selectors

Thanks to Eric Meyer author of Cascading Style Sheets: The Definitive Guide for the following, in which he patiently explains the way CSS pseudo-class selectors work, particularly in conjunction with descendent (contextual) selectors.

Let's say that I want a given style to be applied to the content
of all TD elements. That's written as:

TD {color: red;}

Right, no surprises there. In CSS terminology, the selector 'TD' is
an element selector which matches all elements of that name. Now, if I want to color only those TD elements which have a class of 'navbar' then I write:

TD.navbar {color: red;}

Again, the selector matches any 'TD' element with a class of 'navbar'. Similarly, if I want a selector which matches (or "selects") all unvisited hyperlinks, I write the following:

A:link {color: blue;}

So now I put the two together. I want to select any unvisited
hyperlink which is found within a TD element with a class of
'navbar'. In other words, I only want those hyperlinks which are
descendants of said TD. Thus:

TD.navbar A:link {color: blue;}

The class is still found on the TD element, but I'm actually
selecting the hyperlinks which appear in that context (thus the old
term of "contextual selector"). Let's say I want to select only those unvisited hyperlinks which have a class of 'external'. Thus:

A.external:link {color: green;}

But if I only want to select those unvisited hyperlinks which have a class of external but appear inside TD elements with a class of
'navbar', then we get:

TD.navbar A.external:link {color: lime;}

So what's important is to make sure that you're selecting the right
element for the job you're trying to do. Here's sort of a combined
stylesheet:

BODY {background: white; color: black;}
TD.navbar {background: black; color: white;}
A:link {color: blue; background: white;}
TD.navbar A:link {color: green; background: black;}
TD.navbar A.external:link {color: lime; background: black;}

When I set the backgrounds for the links, I'm really setting
backgrounds on those links, not on the TD or the BODY or anything
else.

- Eric Meyer