That's ok. I actually wanted to include such note about the units in the first place but forgot (in the book I use REMs and pixels only as fallback but changed

That's ok. I actually wanted to include such note about the units in the first place but forgot (in the book I use REMs and pixels only as fallback but changed to pixels in the article for simplicity). Your comment reminded me to add this note in. And I know how frustrating it can be when working on typographic details on the web. Are you having any specific problems?