This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
http://www.whatwg.org/specs/web-apps/current-work/multipage/common-microsyntaxes.html#valid-time-string Point 4 is here to say that: HH:mm is a valid time string because it is implicitely equivalent to HH:mm:00. I would suggest to remove the "(required if second is non-zero)" part. A time string is valid if it is on the following formats: HH:mm or HH:mm:ss or HH:mm:ss.sss. "Valid time string" isn't there to convert the string to a computer-readable time. Only to check if a given string represents a time. Converting a string to a computer-readable time is the role of "Parse a time string": http://www.whatwg.org/specs/web-apps/current-work/multipage/common-microsyntaxes.html#parse-a-time-string This also applies to 4.3.
That part of the spec is giving the conformance requirements for writing a valid time string representing a specific time. If the specific time being represented is one second past noon, then the string "12:00" would _not_ be a valid time string for it.