External CSS contains separate CSS file which contains only style property with the help of tag attributes (For example class, id, heading, … etc). CSS property written in a separate file with .css extension and should be linked to the HTML document using link tag.
This means that for each element, style can be set only once and that will be applied across web pages.
Each HTML page must include a reference to the external style sheet file inside the element, inside the head section
An external style sheet can be written in any text editor, and must be saved with a .css extension.
The external .css file should not contain any HTML tags.