Responsive Webdesign

My first attempt to create a responsive website from scratch

So this is my first real attempt ever to create a responsive website. The code is written by myself and partially adapted from tutorials I watched on Youtube.
In the web development lecture, all students should design their own website within the requirements. 

1 The first task was to build a static website, with fixed navigation. 
2 The second task was to make the website responsive and look good on mobile devices. 
3 Our last task yet was to implement a slider with javascript.

Below you can see the website inside an iframe. bare in mind, that it's not fully functional due to technical issues.
If you find any issues, try to open it on Codepen. I've put a button on top of this side.
Feel free to discover the code. At the top of the iframe, you can navigate through the HTML-, CSS- and Javascript-Code.