Hasn't somebody thought that any material posted under STEM learning material should have at least one reference (preferably books or papers) written along? The purpose of the author who wrote the material is obviously for the material to be of use for the readers who are in need in the related topic. He/she presents it in his/her own way, which is hopefully easier or additionally shorter than the existing literatures, so that the reader can get the grasp quicker. But still that requires reliability and a sense of responsibility, as we don't know the real poster in person, often we only know his username. My experience with materials in STEM was that whenever I came across something that is new to me, first thing that popped in my mind was "can I really take the understanding along this author's way of presenting the matter?". I'm not questioning the credibility of the author, I'm just suggesting that it's better to provide references so that the reader can double check if what he understands from that post alone is sufficient and indeed goes in the same line with the more established source.