A CSS preprocessor is a program that lets you generate CSS from the preprocessor’s own unique syntax.
Advantages of using a css preprocessor is it makes code easier to read, especially on big projects or big websites. Css preprocessor lets you use variables, mixins, functions and also reusability of code. You can also join multiple files into one file, with @import.
Some disadvantages are compilation takes longer time. They can produce very large CSS files. Extra tooling and developer inconvenience. Also debugging is harder.
Sass, Stylus, Less, PostCSS, Pleease.
Flying Hippo - Requires XHTML, CSS, JavaScript, AJAX, XML, JSON, PHP
Cohesive creative and Code - Requires HTML, CSS/SCSS, Javascript, Ruby on Rails, React.js (or PHP, .NET, similar technologies)
Blue Frog Dynamic Marketing - Requires HTML, CSS, Javascript/jQuery
Competitive Edge - Requires ASP, ASP.NET, Microsoft SQL, JavaScript, HTML, CSS
Webspec Design - Requires Git, CSS/SASS, HTML5, JS/jQuery, UI/UX