Use External CSS

Quoted from http://www.tizag.com/cssT/external.php

What's external CSS

When using CSS it is preferable to keep the CSS separate from your HTML. Placing CSS in a separate file allows the web designer to completely differentiate between content (HTML) and design (CSS). External CSS is a file that contains only CSS code and is saved with a ".css" file extension. This CSS file is then referenced in your HTML using the <link> instead of <style>. If you're confused, don't worry. We are going to walk you through the whole process.

Example

This is a html file named index.html:

 1  < html >
 2  < head >
 3  < link  rel ="stylesheet"  type ="text/css"  href ="test.css"   />
 4  </ head >
 5  < body >
 6  < h3 >  A White Header  </ h3 >
 7  < p >  This paragraph has a blue font.  
 8  The background color of this page is gray because
 9  we changed it with CSS!  </ p >
10  </ body >
11  </ html >

Make sure save this file as "index.html" (without the quotes) in the same directory as your CSS file.

Why use external CSS file?

1. It keeps your website design and content separate.
2. It's much easier to reuse your CSS code if you have it in a separate file. Instead of typing the same CSS code on every web page you have, simply have many pages refer to a single CSS file with the "link" tag.
3. You can make drastic changes to your web pages with just a few changes in a single CSS file.