Markdown is popular because it implements just enough features to be useful while not presenting a high threshold for "just writing things down". It exists bec

Markdown is popular because it implements just enough features to be useful while not presenting a high threshold for "just writing things down". It exists because traditionally, people who design markup languages have not done a particularly good job in designing something that is suitable for adding a just enough structure to text to make it useful both as text and as rendered document. (If you want to look at a particularly egregious example of hubris, arrogance and total lack of empathy for users and implementors alike, look at ISO 8879:1986. The cheapest way is probably to get hold of an annotated standard is probably by buying Goldfarb's SGML Handbook. Please burn after reading though - to keep it out of the hands of impressionable kids) I had a long discussion about using HTML+CSS with one of the designers of CSS many moons ago, and sure, in theory we could have used that. But you need only do a "view source" on any web page you regularly visit to realize that it is anything but low threshold.