Task list items (extension)

GFM enables the tasklist extension, where an additional processing step is performed on list items (opens new window).
A task list item (opens new window) is a list item (opens new window) where the first block in it is a paragraph which begins with a task list item marker (opens new window) and at least one whitespace character before any other content.
A task list item marker (opens new window) consists of an optional number of spaces, a left bracket ([), either a whitespace character or the letter x in either lowercase or uppercase, and then a right bracket (]).
When rendered, the task list item marker (opens new window) is replaced with a semantic checkbox element; in an HTML output, this would be an <input type="checkbox"> element.
If the character between the brackets is a whitespace character, the checkbox is unchecked. Otherwise, the checkbox is checked.
This spec does not define how the checkbox elements are interacted with: in practice, implementors are free to render the checkboxes as disabled or inmutable elements, or they may dynamically handle dynamic interactions (i.e. checking, unchecking) in the final rendered document.

Example 279

Markdown	HTML	Demo
- [ ] foo - [x] bar	<ul> <li><input disabled="" type="checkbox"> foo</li> <li><input checked="" disabled="" type="checkbox"> bar</li> </ul>

Task lists can be arbitrarily nested:

Example 280

Markdown	HTML	Demo
- [x] foo - [ ] bar - [x] baz - [ ] bim	<ul> <li><input checked="" disabled="" type="checkbox"> foo <ul> <li><input disabled="" type="checkbox"> bar</li> <li><input checked="" disabled="" type="checkbox"> baz</li> </ul> </li> <li><input disabled="" type="checkbox"> bim</li> </ul>