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.
CSS separates content contained in the HTML file from how the content should be displayed. That separation between content and how to display it enables devices to render the content differently based on factors such as screen size. For example, a mobile device does not have as much space to display a navigation bar on the side of a page so it is often pushed down below the main content. The Bootstrap Blog example shows that scenario when you resize the browser width.
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.
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.
Responsive design is an approach for creating CSS that lays out content differently based on screen attributes. The responsiveness is accomplished by implementing media queries in the CSS.
CSS frameworks provide structure and a boilerplate base for building a web application's design.
The Bootstrapping Design book is one of the clearest and concise resources for learning design that I've ever read. Highly recommended especially if you feel you have no design skills but need to learn them.
Kuler is a complementary color picker by Adobe that helps choose colors for your designs.
Mozilla Developer Network's CSS page contains an extensive set of resources, tutorials and demos for learning CSS.
CSS Positioning 101 is a detailed guide for learning how to do element positioning correctly with CSS.
Learn CSS layout is a simple guide that breaks CSS layout topics into chapters so you can learn each part one at a time.
