Class and ID Selectors

Any element can have a class attribute:
```
<p class="lie">This is a lie</p>
```
whose value can be used as a selector:
```
.lie {color: purple;}
```
Similarly, any element can have an id attribute:
```
<p id="thebiglie">I will have it by tomorrow</p>
```
whose value can be used as a selector:
```
#thebiglie {color: purple;}
```
ids should only be used one per document, while class can be used many times.
Pseudo-classes represent run-time state. They are:
- :link a hyperlink that has not been visited.
- :visited a hyperlink that has been visited.
- :active a hyperlink that is being clicked on.
- :hover an element over which the mouse is pointing.
For example:
```
a:visited { color: black}
a:active  { color: red }
a:hover { background: #ffa }
```
The selector can also be contextual:
```
h1 em {color: blue;}
```