Step-by-step explanation:
When discussing the advantage/dis- of CSS, its important to note the two types of CSS
Inline and External CSS.
Inline CSS allows for on the spot changing of the style of whatever.
External CSS allows for site wide changes to the style of whatever.
When it comes to UX, External CSS creates a longer load time of the webpage, especially if there's a large amount of styles for things.
Inline CSS causes problems when you want Javascript oriented design as Javascript overwrites Inline. when coded in a specific way.