Here's a list of free courses that I took in the past and currently taking that I think you should take to have a better grasp of web development. Most of these courses are introductory courses since they are free but nonetheless, having a good foundation can help you tremendously in your career so I highly recommend that you at least take one or two courses from this list.

1. Programming Languages

This is a beginner introductory course about theory and practices of programming languages.
https://classroom.udacity.com/courses/cs262

2. Version Control with Git

If you want to learn git this is the course for you.
https://classroom.udacity.com/courses/ud123

3. Responsive Web Design Fundamentals

At the end of this course, you'll have all the tools needed to build a responsive website.
https://classroom.udacity.com/courses/ud893

4. Responsive Images

This course is all about making your images responsive. I'd recommend that you take it after finishing Responsive Web Design Fundamentals.
https://classroom.udacity.com/courses/ud882

5. HTML5 Canvas

Learn how to create images and interactive animation with HTML5 Canvas.
https://classroom.udacity.com/courses/ud292

6. ES6

Everything you need to know about javascript ES6.
https://classroom.udacity.com/courses/ud356

7. Web Accessibility

Your website should be accessible to everyone and in this course, they show you how you can do it.
https://classroom.udacity.com/courses/ud891

8. Writing READMEs

Documentation is an important part of the development process and in this course, they teach you how you can write good READMEs.
https://classroom.udacity.com/courses/ud777

9. Intro to Algorithms

If you want to get started with algorithms this course is for you.
https://classroom.udacity.com/courses/cs215

10. Intro to Progressive Web Apps

Learn how you can build a PWA.
https://classroom.udacity.com/courses/ud811

If you know of other free courses on Udacity share them in the comments!