I can't recall ever seeing three hyphens used to signify an em dash in a document published on the net. (I added the word "published" to account for the possibi

I can't recall ever seeing three hyphens used to signify an em dash in a document published on the net. (I added the word "published" to account for the possibility of the existence of a markup language I am unaware of that employs your three-hyphen convention.) I am fastidious enough about usage that I probably would've noticed and remembered. In my experience, its always two hyphens (or space, hyphen, hyphen, space).