The first step in https://drafts.csswg.org/css-flexbox/#intrinsic-main-sizes has a fairly complex second sentence. I had to read it twice to note that the parentheticals referred to the same result in the first clause. Scanning quickly I assigned the parentheticals to their closest sentence part. I think my problem is that the parenthetical came after the preposition.

In other words, in this construction "if the result is not zero, divide it by {(some parenthetical) its foo}" I'm scanning {} as a prepositional phrase that's the object of the verb. It took me a bit to notice the word 'result' and fix my confusion.

I think it would be clearer to say "if the result is not zero, divide a positive result by … or a negative result by …"