Being someone who was ahead of the game in css 15 years ago, but not having touched it since - where would one go to understand the new fundamentals and options available?
And then follow it up with this one if you want to go a little deeper: https://css-tricks.com/flex-grow-is-weird/
Personally, I think flexbox is the single most versatile and important tool to be familiar with. You can create nearly any layout with it, in such a way that it's very naturally responsive almost by default.
Throwing out blanket statements is dangerous for new comers, which seems to be the target audience of this article according to its title. For example, collapsing margin isn't always bad and unpredictable, it just seems that way if you didn't take the time to understand the fundamentals behind and go for "what works".
That's not too different to telling CS students to not worry about learning the memory and time complexities of different algorithms and just use [insert "best" algorithm] because it works most of the time.
These are actual practical usable fundamentals.
Complement that with http://inclusive-components.design/
Her explanation of positioning, responsive design and the links to other resources she provides inside are all good.
These days, as mentioned in the OP, MDN is the best point of call for checking syntax or learning how to implement something new the right way.