Wow, that's quite an achievement when you made it all the way through the tutorial and created your own portfolio project!
I will show you some ways on how you can further develop your skills as a web programmer.
More HTML & CSS
The More HTML & CSS series is a collection of instructions on various topics related to HTML and CSS. Just look at the topics that you are interested in.
Web Applications with Dart
Learn to program dynamic web applications in Dart. Hello Dart offers an introduction that does not require any experience in programming.