These errata have the same status as a Working Draft.
[2013-07-23] ':nth-child', 'nth-last-child', 'nth-of-type', nth-last-of-type', 'first-child'. 'last-child', 'first-of-type', 'last-of-type', 'only-child', and 'only-of-type' can match elements without a parent and may also be used to select roots in an ordered list of trees. E.g., ':root:nth-child(1)' matches the same element as ':root'.
[Updated text for “6.6.5. Structural pseudo-classes” forthcoming.]