As an alternative, the todo.txt format [1][2] first defined in 2006 [3] is supported by a CLI client (desktop) and several GUI clients (desktop, mobile, web).
This is what I use. It's a little hobbled in terms of add-ons to the standard because the orginal standard isn't centralized or versioned, but it works really well for a lot of things.
[1]: https://github.com/todotxt/todo.txt
[2]: http://todotxt.org/
[3]: https://web.archive.org/web/20060701121920/https://lifehacke...