What is CSS?
The full form of CSS is Cascading Style Sheet. It is a standardized styling format for HTML and websites. It is good practice for websites to use an external style sheet based in CSS so that the site can render the correct styles on it.
For instance if you design a 12 page site in HTML code without an external CSS file and then for some reason if you want to make any changes in terms of navigation you cannot make the change to just one page but would rather have to make the changes physically to each page .
But you can avoid this by using CSS because it stores all the styles of your site in one single external file.