1. An optional U+FEFF BYTE ORDER MARK (BOM) character.
2. The string "WEBVTT".
3. Optionally, either a U+0020 SPACE character or a U+0009 CHARACTER
TABULATION (tab) character followed by any number of characters that
are not U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters.
4. Exactly one WebVTT line terminators to terminate the line with the file
magic and separate it from the rest of the body.
Issue: #580
a1ab1fc2a2