eg. "ch" instead of "ĉ".
It's less common than the x-system of typing accented characters, but some Esperanto sites do have it as an option, such as http://bertilow.com/.
It seems to introduce ambiguity, because Esperanto already has a letter h. The entire meaning of words might be changed, one word turned into another word.
Why was "h" chosen for this purpose? Are there words that can be "broken" like this, by being typed with the h-system?