Yesterday, I attended a training on Sass and Compass by Roy Tomeij organised by Fronteers. Sass is a CSS meta language and compiles to CSS, to make it easier to maintain than CSS yet still usable in the same browsers.
Compass is a plugin for Sass and offers a lot of often used patterns, but it does more, which I'll explain below.
Sass is a front-end meta language (or CSS preprocessor) that compiles to CSS. It has 2 syntaxes, "Original Sass" and SCSS. SCSS is easiest to start with if you already know CSS, because it just adds syntax. "Original Sass" and other CSS meta languages like LESS are out of scope here. How to run Sass is also out of scope, but let me just say that there is a free utility called Scout that makes it easier. However, compiling on the command line is a lot faster. For debugging in the browser you can try FireSass or enable line numbers when compiling Sass.
As mentioned, it's better if you already know CSS. As anyone who ever used Frontpage knows, preprocessors can result in bloated code. That's why it's recommended to monitor the generated CSS, especially in the beginning. You'll need to learn how the Sass compiler works and that way you can make really clean code.
Although the CSS3 spec is very much alive and new features that make the lives of developers easier are added regularly, it takes time for all browsers, particularly those that don't push new versions coughIEcough, to support these. For some of these features, Sass (e.g. in combination with the Compass CSS3 module) is a good workaround, because it results in code that works on current browsers and it is easy to incorporate fallbacks.
So far I've only argued why Sass is a good idea, now I want to show that it's really easy to use and how you can profit from it as a developer. I won't go into all topics discussed in the training, but I'll add a list of links of the basic concepts in the Sass documentation.
An annotated example:
This compiles to:
Compass is a library for Sass, and a bit more. It adds:
Because Sass is a compiler, it will check the input for syntax errors. The compiler can even be configured to do minification. This will concatenate the included files (which is better for performance, because there is only 1 HTTP request), compress the result and also remove all comments, except those starting with /*! which is conform other minifiers.
It is advised to group all variables for e.g. colors in one file and all mixins in files separated from the other code. It is easy to make a modular set-up in Sass, because you can include other Sass files that are automatically combined to one file. Because the architecture of a web project is beyond the scope of this post, I want to refer to the book that Roy is writing on this subject. He also has a presentation about modular and reusable front-end code. Another way to learn more about CSS architecture is to be guided by Smacss.