settings, size
<link rel="help" href="https://w3c.github.io/webvtt/#collect-webvtt-cue-timings-and-settings">

assert_equals(cues.length, 8);

var valid_vertical = [
    '',
    'lr',
    'rl',
    'lr',
];
valid_vertical.forEach(function(valid, index) {
    assert_equals(cues[index].vertical, valid, 'Failed with cue ' + index);
});

for (var i = 0; i < 4; i++) {
    var index = valid_vertical.length + i;

    assert_equals(cues[index].vertical, '', 'Failed with cue ' + index);
}

===
WEBVTT

00:00:00.000 --> 00:00:01.000
text0

00:00:00.000 --> 00:00:01.000 vertical:lr
text1

00:00:00.000 --> 00:00:01.000 vertical:rl
text2

00:00:00.000 --> 00:00:01.000 vertical:rl vertical:lr
text3

00:00:00.000 --> 00:00:01.000 vertical:
invalid4

00:00:00.000 --> 00:00:01.000 vertical:RL
invalid5

00:00:00.000 --> 00:00:01.000 vertical: rl
invalid6

00:00:00.000 --> 00:00:01.000 vertical:vertical-rl
invalid7