According to the spec, the payload of a WebVTT cue block consists of one of three options, the first one being WebVTT caption or subtitle cue text, which, in turn, consists of "zero or more WebVTT caption or subtitle cue components, in any order, each optionally separated from the next by a WebVTT line terminator."

Therefore, an empty cue block is valid. In practice, I've encountered several Youtube videos that have such empty cue blocks.

To modify an example from the test cases, the following valid WebVTT file fails to parse:

WEBVTT

00:01.000 --> 00:04.000
- Never drink liquid nitrogen.

00:04.000 --> 00:05.000


00:05.000 --> 00:09.000
- It will perforate your stomach.
- You could die.

Attempting to parse this yields the following error:

ParseError { location: LineCol { line: 7, column: 1, offset: 88 }, expected: ExpectedSet { expected: {"[' ' | '\\t']", "['0' ..= '9']"} } }