Last year or so, I worked on a platform where I would publish video courses. The first one is on web fundamentals (in Bulgarian), and it's almost ready. You can check it out here. But this article is not about that. It's about a platform feature I built - an in-house validator to exercise the gained HTML, CSS, and JavaScript knowledge. After each lesson, I give a task to the students, and they can work on it right in the browser. The small application became an inspiration for another project - iloveweb.dev. So, I decided to share how the validation works.