These are so called because the behavior is as if the element was specially marked as that class. Note that pseudo-class selectors that cause the document to reflow may be ignored by web browsers. An example of this would be a different font size on a visited link to an unvisited one. A link cannot be both visited and unvisited. This applies to an element being activated.
Links can be active as well as linked or visited.
This is a very important concept indeed. Selector grouping In order to associate a block with several different selectors, and therefore save typing and download time, one can separate the selectors by commas. The next section deals with lengths, how conflicts between style declarations are resolved, etc. See copyright document for terms of use.