CSS is the primary language used to style the web. Combined with HTML and other web technologies, it gives you full control over how things look on your website.
(www.codecademy.com)
With the fundamentals of HTML in your back pocket, take your skills to the next level with CSS and styling. This interactive Codecademy course gives you an overview of what CSS and HTML can do together.
(mdo.github.io)
There are a number of style guides you can find on the Internet. Here's one that's easy to follow by the co-creator of Bootstrap.
(jgthms.com)
This is an amazing demonstration of how to use CSS to build a basic webpage. It gives you all the fundamental building blocks of a well designed page step-by-step.
(learnlayout.com)
Putting CSS to work to practically create layouts for a webpage isn't exactly obvious from just learning the rules. Learn how to use positioning properties in CSS to organize and structure the content of your website.
(css-tricks.com)
A lot of the time you will be using the work of others when writing CSS. This pre-written CSS you can use are known as CSS frameworks. Learn about what they are and what their benefits are.
(getbem.com)
Once you start working on very large projects with a lot of CSS in them, you will need to learn how to properly name and organize your styles. Block-Element-Modifier (or BEM) is a practice of how to organize your styles. Read this introduction to learn one method of keeping your CSS manageable.
(cssreference.io)
This is a comprehensive and visual guide to CSS properties. Give it a quick skim and keep this in your back pocket when you need to look something up in CSS.