>>81815487web pages are composed of three parts:
markup, which defines what is being shown. you write this in HTML. this includes things like text, pictures, links, and blocks.
stylesheets, which defines how things are shown. you write this in CSS. this includes things like font color, background color, borders, width, etc.
scripts, which define how things interact with the user, the server, and other things. you write this in javascript. this includes things like click events, ajax, etc.
youtube provides a chat window, and a stylesheet can overrides how that window looks, stuff like changing its color, font, etc.
