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The Conformance section (2.1) encourages authors to use a validator but does not define what a validator is. Note with reference to github/w3c/webvtt issue #217 (https://github.com/w3c/webvtt/issues/217#issuecomment-145750657) that the intention is for different behaviour in validators vs user agents. If this is the case then the concept of validation needs to be defined explicitly with separate requirements to those for UA presentation.
https://github.com/w3c/webvtt/pull/255