Still not usual. The lists I use to use contain all elements. So in certain cases, it might make no sense to talk about light elements, or it's due to layout reasons, whatsoever. The fact, that you ask, makes me think, you suggest some underlying, hidden meaning. There is none. If someone created a list which starts with oxygen, so it's this someone whom you should ask. Otherwise it's simply guesswork about the author's intentions.I have read about 10 such articles.