If you learned CSS before, you should know that CSS is not a programming language. You can use it to design webpage style, but you cannot use it for programming, i.e, CSS is what designer uses, not what programmer uses. Programmer may think that CSS is very troublesome, it has no variables, no conditional statements, it just allows line-by-line description of HTML elementsLuckily, CSS preprocessor appear which makes CSS programmable. The general idea of CSS preprocessor is using a programming language to design the website style, and then convert it to CSS files.Among different CSS preprocesso...