When using Cascading Style Sheets(CSS),it is advisable to use External Style Sheets. 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 instead of .

Placing CSS in a separate file has the following advantages:

• It allows the web designer to completely segregate content(HTML) and design (CSS).
• 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.
• Major changes can be made to your web pages with just a few changes in a single CSS file.

However,even if your using the external CSS, a minute change in syntax may not lead the CSS to be applied to the HTML file.

Here is one scenario discussed in htmlforums.com/css/t-resolved-external-style-sheet-not-linking-properly-120616.html.Check the response and the trivial syntax error in the CSS file.