How could you improve this code? | Fixed Navigation Bar | HTML, CSS & JS

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View code on GitHub:
github.com/dco...
In this video I have a go at creating a fixed navigation bar from scratch using bare-bone HTML, CSS and JavaScript. I'd now like you guys to give your opinions on the code and suggest any possible ways of improving this in terms of speed, characters used or general "good practice".
I'm doing this in hopes of creating a discussion-style environment where everyone can learn from each other and we can all contribute, making it an interactive experience.
I also wanted to do something slightly different on the channel where you can see how I would code something from scratch.
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[@wswebus922]

Could you put your code on Codepen or Github?
Thank you

[@dcode-software]

Good idea, especially for these larger type of videos - thanks mate!

[@dcode-software]

I've uploaded this code onto GitHub:
github.com/dcode-youtube/fixed-navigation-bar-feedback/