hover/active link styles cannot work?

When we set the link styles in CSS, like:
< style type="text/css" >
<!--
a:link { color:red }
a:visited { color:green }
a:active { color:orange }
a:hover { color:blue }
-->
</style>

but "hover/active" link styles may not work in sometime?

a simple solution is change the sequence when you write the CSS:
1. Link
2. Visited
3. Hover
4. Active