You can’t call yourself a developer without solid HTML and CSS skills. It’s common even as a regular webmaster to be familiar with them. You will impress no one by mentioning your intermediate HTML and CSS skills in your CV. Altogether, this isn’t correct because developers who ace HTML and CSS are pretty few. HTML, and especially CSS, are hugely complex, and it takes years to master them like a pro.
For instance, a top-class CSS expert knows how to replace JavaScript effects with CSS code. This skill is golden for WordPress theme development because it reduces the theme filesize.
You can’t go wrong by mastering CSS! It’s relatively easy to learn the fundamentals, but it’s more challenging to be a genuine CSS expert. The following resources are all you need to improve your CSS skills.
W3Schools – CSS Tutorial

W3Schools is one of the oldest resources for aspiring developers. It’s a learning hub covering a wide range of topics, from HTML, CSS, and responsive design to machine learning, data science, Python, and Amazon Web Services.
The CSS tutorial is comprehensive and students have to complete practical exercises, so it’s not enough just to read simple explanations. Moreover, you may enroll in the course and take an exam to prove your skills. A certification issued by W3Schools is a solid piece of evidence that you are a skilled CSS practitioner.
Awesome-CSS Learning

This GitHub repository is a great resource for beginners and experts alike. It includes everything you should know to become a great CSS specialist. It starts with the fundamentals and ends up with complex animations and transitions. Practically, this repository is a collection of resources, ordered from simple to complex. You save time by following the created path since the content has been curated by a professional.
Learn CSS

Learn CSS is the perfect resource if you are new to CSS or want a short recap. Google experts created Learn CSS, so you learn from the best by joining this class. This course covers all the aspects related to CSS. Yet, the Web Dev library includes other adjacent classes like HTML, Responsive Design, Forms, PWA, Accessibility, Images, and Privacy.
The only downside is that you don’t have any practice section. However, it compensates with a large set of examples.
HTML & CSS Courses on Codecademy

Codecademy is a complex online learning platform providing classes on a wide range of topics like web development, programming, cybersecurity, DevOps, machine learning, and data science. A few classes are about CSS and they should be on your radar because the theory and practice are blended together to help students learn efficiently. Certainly, you won’t waste your time on Codecademy.
Cascading Style Sheets by Mozilla

The Mozilla documentation is pretty plain and looks daunting for beginners. However, it fully deserves its place on this list thanks to the comprehensive and complete format. Visit this website if you want to learn the ins and outs of each CSS property. The same as the previous course, this guide doesn’t include a practice section but has plenty of relevant examples.
Responsive Web Design Certification by freeCodeCamp

freeCodeCamp is a huge resource for aspiring developers. It has modules for basic web development technologies like HTML, CSS, and JavaScript, but also for trendy domains like machine learning, Python, MongoDB, Bash, and Node.js. Among the unique features of freeCodeCamp are its focus on practice and the integrated approach to web technologies. No one became a pro by watching tutorials. Practice is the key! Also, learning CSS alone isn’t quite helpful. It’s way better to learn practically how CSS styles HTML elements.
CSS-Tricks

CSS-Tricks is the kind of blog you have to check periodically to learn what’s new in the industry and improve your CSS skills. You’ll find mostly in-depth articles about CSS here, but you’ll also find posts about web development, React, freelancing, JavaScript, and WordPress in the archive.
The articles are well-written and full of valuable information. CSS-Tricks is one of the oldest blogs treating CSS topics, so the quality of the content is out of the question. I recommend visiting the Guides section because there are tens of complete articles about flexbox, media queries, and dark mode on the web.
Smashing Magazine

Smashing Magazine is a huge authority in the web development blogging industry. It started back in 2006 and since then millions of developers and designers get inspired by Smashing Magazine articles.
It’s not a pure CSS resource because Smashing Magazine is about web development and design. Yet, there are many interesting articles related to CSS. What’s really important is that only top-class experts write for this publication. You learn from the best in the world, so you shouldn’t skip visiting this blog.
CodePen

Practice is crucial for learning a language and CSS isn’t an exception. CodePen is a social development environment massively used by millions of developers. In plain English, CodePen is a place where users may write code directly from the browser, save the snippets, and share them with fellows. CodePen focuses on front-end languages like HTML, CSS, and JavaScript. Therefore, you can use CodePen to write CSS code, get inspired by other users, and share your work to impress people.
Frontend Masters Online Bootcamp

Frontend Masters is a famous resource for frontend developers thanks to the quality of the classes and workshops. Fortunately, you may freely enroll anytime in an introductory bootcamp for frontend developers. This bootcamp is made up of the following classes:
- Introduction to HTML
- Introduction to CSS
- HTML Forms
- Website Embeds & GitHub Pages
- Calculator Project: HTML & CSS
- Introduction to JavaScript
- Calculator Project: JavaScript
- Using JavaScript in Websites
- Build a Game Project: Feed-A-Star-Mole.
I purposely put this resource in the last position because it’s not necessarily a place to improve your CSS skills. However, learning CSS alone is almost useless. You learn CSS to use it in web development and this bootcamp is the ideal website to start your career. Pay attention that the bootcamp is 20 hours long, so you have to allocate enough time on your schedule to watch the videos.
Over to You
The above resources to master your CSS skills cover all types of users and requirements. Some of them are online courses while others are blogs for CSS enthusiasts and developers. A part of the suggestions are for aspiring frontend developers while others are for genuine experts. Hopefully, you will find at least a useful recommendation for you!