In reply to Mary from London.

Well, I am not able to reach the benefits of this argument about how we should promote ease of use by exposing less features to users in the editor.

The article is asking where we draw the line in terms of the number of global styles options, not whether exposing fewer features promotes ease of use (although, that is a worthy topic to explore, which has been written about extensively based on user testing). There will be a line. You can count on it. Someone will decide what it is. It is best we begin exploring the topic before that decision is made. I imagine that line will see some adjustment over time, but it will be there, at least initially.

In the same paragraph, I read two contrasting sentences:
“For example, most users should have no need to adjust the line-height for their text.”
(…)
“At a certain point, it is better to learn CSS.”

Those two sentences were meant to be complimentary, but we're missing the context of the entire paragraph by picking them out. Nevertheless, once you get to a certain point with options that the majority of users do not use, the user experience degrades. Going back to "the line" I mentioned above. Someone will make a decision along the way that says, "Here are the options we'll offer; the rest of the stuff will be left to custom CSS." And, there is always that user who will need to do something more than what the editor provides, regardless of where the line is. When a user gets to that point, the only option left is CSS (or another editor, as the case may be).

I used line-height as an example because I do not think that is a good v1.0 option. Provide some font family and font size choices. See how that works out. Test a solution that automatically calculates line-height (based on work I've done in the past, this has done well). Test a solution that provides the choice to the user. Get feedback. Determine the best course of action that mixes user choice with good design principles.

How can the proposal of learning a code language, even if its basics, through CSS, be better as an solution for the casual users, rather than allowing them to simply choose a value in a checkbox?!...

The article does not argue that users should learn to code CSS over selecting options. I would be happy to clarify anything I wrote. There seems to be a misunderstanding here that forms the basis for the remainder of your reply, so I'll just leave things here. I'd be happy to discuss. I just think we're on different wavelengths here.