How to use/implement CSS

CSS can be implemented in three ways in an HTML document(webpage):

Inline CSS

To style a specific HTML element, Inline CSS is used. To implement this use style attribute.

<p style="background-color:yellow;">Hello World</p>

To Style HTML elements of a single webpage. It is implemented with inside head section.

<head>
<style>
body {
    background-color: green;
}
</style>
</head>

To style multiple webpages of a website with a single CSS file. External stylesheet reference must be added using tag in the head section.

<head>
<link rel="stylesheet" type="text/css" href="stylesheet.css">
</head>