MDN's docs are amazing, but they are about everything. "CSS tricks" is not documentation (except some very helpful articles).

Meanwhile in Tailwind everything is grouped together, can be searched quickly and acts both as quick docs [1] and as quick reference [2].

I personally have become proficient with flexbox by playing with Tailwind classes.

[1] E.g. layouts https://tailwindcss.com/docs/display

[2] E.g. all the justify-content values https://tailwindcss.com/docs/justify-content