Cascading Style Sheets (CSS)

Cascading Style Sheet (CSS) files contain rules for how to display and lay out the HTML content when it is rendered by a web browser.

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Why is CSS necessary?

CSS separates the content contained in HTML files from how the content should be displayed. It is important to separate the content from the rules for how it should be rendered primarily because it is easier to reuse those rules across many pages. CSS files are also much easier to maintain on large projects than styles embedded within the HTML files.

How is CSS retrieved from a web server?

The HTML file sent by the web server contains a reference to the CSS file(s) needed to render the content. The web browser requests the CSS file after the HTML file as shown below in a screenshot captured of the Chrome Web Developer Tools network traffic.

Google Chrome Web Developer Tools shows how CSS is separate from the HTML content.

That request for the fsp.css file is made because the HTML file for Full Stack Python contains a reference to theme/css/fsp.css which is shown in the view source screenshot below.

View source screenshot for the fsp.css file in index.html.

CSS preprocessors

A CSS preprocessor compiles a processed language into plain CSS code. CSS preprocessing languages add syntax such as variables, mixins and functions to reduce code duplication. The additional syntax also makes it possible for designers to use these basic programming constructs to write maintainable front end code.

Sass is currently the favored preprocessor in the design community. Sass is considered the most powerful CSS preprocessor in terms of advanced language features.
LESS is right up there with Sass and has an ace up its sleeve in that the Bootstrap Framework is written in LESS which brings up its popularity.
Stylus is often cited as the third most popular CSS preprocessing language.

CSS preprocessor resources

The Advanced Guide to HTML and CSS book has a well-written chapter on preprocessors.
Sass vs LESS provides a short answer on which framework to use then a longer more detailed response for those interested in understanding the details.
How to choose the right CSS preprocessor has a comparison of Sass, LESS and Stylus.
Musings on CSS preprocessors contains helpful advice ranging from how to work with preprocessors in a team environment to what apps you can use to aid your workflow.

CSS libraries and frameworks

CSS frameworks provide structure and a boilerplate base for building a web application's design.