CSS(Cascading style sheets) is the second major language of the web. 
Cascading Style Sheets (CSS) is a styling language. It is the language used to style markup languages e.g HTML documents. CSS is an essential language in web development alongside HTML and JavaScript.
CSS can be used to style HTML documents in three ways: inline, internal and external.
The inline CSS is written on the particular HTML tag, styling a single HTML property.
<ADD IMG>
The internal CSS consist of the complete styling of a HTML document written in the HTML file.
<ADD IMG>
The External CSS is written completely in a .css file, with linking from the HTML file. External CSS is most recommended when coding because it makes your work cleaner and neater.