External Style Sheet (link to a style sheet) :
The method to link html with style sheet is called external style sheet.
An external style sheet is a text file with the extension .css. Like other files, we can place the style sheet on your web server or hard disk.
For example : save the style sheet with the name style.css and place it in a folder named style.

To create a link from the HTML document (default.htm) to the style sheet (style.css). The following code will be inserted in the header section of the HTML code i.e. between the <head> and </head> tags. HTML file.
<link rel=”stylesheet” type=”text/css” href=”style/style.css” />
The code will be as follows :
Default.htm
<html>
<head>
<title>My document</title>
<link rel=”stylesheet” type=”text/css” href=”style.css” />
</head>
<body>
<h1>My stylesheet Page</h1>
</body>
</html>
style.css
body {
background-color: #FF0000;
}
This link will display the layout from the CSS file in the browser when displaying the HTML file.
Output of the above code will be as follows :

One CSS file can be used to control the layout of many HTML documents. Using CSS, the change can be made in a few seconds just by changing one code in the central style sheet.