CSS can be written in three different ways for an HTML file. 

1. Inline: Writing CSS for an element along with the element definition using the style property.
2. Embedded: Writing the CSS in the head using the style tag.
3. External: Having CSS written in an external document and linking it via LINK tag.

The best way to write CSS by having an external document and linking it in the REQUIRED HTML documents. Whereas the reason why having an external document is best because having our CSS written inline it should be rewritten for ANOTHER element too lots of same code comes up. Whereas CSS is embedded in a style tag we cannot use the styles in a different HTML document again the code should be rewritten in different HTML document too. So, the best way is to use an external document to write CSS and link it in an HTML document.

Ex:

1. Inline: <h1 style=”color: RED;”>Hello, World!</h1>
2. Embedded: <style>h1{color: red;}</style>
3. External: <link rel=”stylesheet” type=”text/css” href=”main.css”>

The inline one is written along with the HTML element encapsulated in the body tag. And, the embedded style is written in the style tag encapsulated inside the head tag. And, the external style sheet named main.css is LINKED using link tag.