Lists with checkboxes that GitHub takes seriously
GFM task lists are unordered list items prefixed with [ ] (incomplete) or [x] (complete). The bracketed bit must come right after the list marker, with a single space.
What changes on GitHub
On issues, PRs, and discussions, task list items render as interactive checkboxes. Anyone with write access can toggle them. Each toggle becomes a real edit to the issue body — there is an audit trail in the issue's edit history.
GitHub also computes a completion percentage from task lists in PR descriptions. PR list views display '3 / 8' next to the title. Cheap progress bar for free.
Nesting tasks
Indent the same way you nest any list: 2-space increments. Nested tasks render as nested checkboxes; the parent is independent of children (toggling a parent does not toggle children).
Principle: A task list in an issue is a contract with the team. Each box is one verifiable, atomic step. Vague items ('improve performance') don't deserve a checkbox; concrete ones ('add index on users.email') do.