CSS is a core technology of the web and is a must know for everyone who wants to make web applications.
You can read more about css here.
When doing web development responsive design is a standard practice and it means that your layout should look good on any device a user may use to visit your web application.
You can read more about responsive design here.
The following is a list of the main css architectures and you should be aware of them:
CSS will behave in different ways in different browser so it is important to understand how to make your styles work in every browser.
A good starting point is to learn about vendor prefixes.
Preprocessing css is standard practice for most projects and it allows you to use features that are not part of standard css, the following is a list of the most common preprocessing styles.
The following are some of the most popular css frameworks and you are very likely to come across them when working.
The following is a list of useful resources to learn more about css.