Syntax
Start a list item with a space inside brackets for an unchecked box, or an x for a checked one: - [ ] todo, - [x] done. There must be a space between the dash and the bracket and after the closing bracket.
Where the boxes are clickable
In GitHub issues, pull requests and comments you can tick boxes directly and GitHub rewrites the Markdown for you; it also shows progress like “2 of 5 tasks”. In READMEs and most other renderers they are read-only.
Nested tasks
Indent sub-tasks by two spaces (under -) or three (under 1.).
Checkboxes in a table
Task-list syntax only works in lists. In a table cell, use an emoji such as ✅ / ⬜, the shortcodes :white_check_mark: / :white_large_square:, or HTML <input type="checkbox" disabled checked> where allowed.
Uppercase X
[X] also counts as checked on GitHub and in markdown-it.