Sweet. I like the two size options. This will make stories much more readable. I can’t help feeling like I’m partly responsible for this improvement…? Maybe not.
One thing though, I’m not sure why you chose to attach the section heading to the following paragraph without a space (ie, <br> instead of <p>). That’s not something I’ve ever seen in any other publications. It looks OK when you use both sizes together like in the examples, but when you use just a single heading it looks kind of weird stuck to the text like that.