I have just tried htmd-cli. As a *nix user, I expect command-line tools that process text to act as filters by default: to read from standard input and to write to standard output. That htmd-cli doesn't do this is surprising. It is different from related tools like Pandoc and cmark. As a result, there seems to be no easy way to test bidirectional conversion with cmark < test.md | cargo-htmd, for example.

Please make reading from standard input and writing to standard output the default. To be consistent with others, the program should write to standard output even when given a file as input.

It would also be good to interpret the filename - as standard input/output. This is a widespread convention.