I can contribute an example.
All the “tick” symbols in the tables in your tutorial are coloured black in my phone’s browser (Android Chrome), but on my PC (Windows Chrome) some of them are coloured as red.
The reason for the discrepancy is that on my Android device the tick symbol is being interpreted as an emoji (i.e. a picture-based glyph) and the colouring is determined by the platform (e.g. https://emojipedia.org/check-mark/ ).
Emojis (as currently implemented) break the concept of text that can have its colour styled with CSS!