CSS stands for Cascading Style Sheet and is a standardized styling format for websites and HTML. It is recommended that websites always use an external style sheet based in CSS in order for the site to render the correct styles on it.
If, for example, you developed a 10 page site in HTML code without an external CSS file, then decided to change your navigation slightly to a different colour or position, then you’d have to change every page on your site which could take a long time.
CSS avoids this since all the styles of the site are held in one single external file.
