Hi Artur!
I'm a beginner myself and I also struggle with units, so I'll just share what I know as best practice.
When it comes to fonts, it's better not to use px. Here's an excellent article about it, and what to use instead (I believe it was shared on the discord group): Why you shouldn't use pixels for font-size: https://fedmentor.dev/posts/font-size-px/
Congrats on completing the challenge! :)
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