The string "WEBVTT".
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.
This commit will now make it ignore a block if it contains zero or more characters on the line starting with WEBVTT.
Here is one of our chapter files, which passes validation, but fails to be parsed properly: http://ads.thisoldhouse.com/chaptest/TOH3021_chapters.vtt
It just reads the first line as an empty block.
According to the standard:
https://www.w3.org/TR/webvtt1/#file-structure
This commit will now make it ignore a block if it contains zero or more characters on the line starting with WEBVTT.