The external style sheet is generally used when you want to make changes on multiple pages. It is ideal for this condition because it facilitates you to change the look of the entire web site by changing just one file.
It uses the <link> tag on every pages and the <link> tag should be put inside the head section.
The external style sheet may be written in any text editor but must be saved with a .css extension. This file should not contain HTML elements.
Let's take an example of a style sheet file named "mystyle.css".
Note: You should not use a space between the property value and the unit. For example: It should be margin-left:20px not margin-left:20 px.