whitespace chars
<link rel="help" href="https://w3c.github.io/webvtt/#collect-a-webvtt-block">

assert_equals(cues.length, 3);

assert_equals(cues[0].id, 'spaces');
assert_equals(cues[0].text, '   text0');

assert_equals(cues[1].id, 'tabs');
assert_equals(cues[1].text, 'text1');

assert_equals(cues[2].id, 'form feed');
assert_equals(cues[2].text, 'text2');

===
WEBVTT

spaces
   00:00:00.000    -->  00:00:01.000\x20
   text0

tabs
\t\t\t00:00:00.000\t\t\t\t-->\t\t00:00:01.000\t
text1

form feed
\f\f\f00:00:00.000\f\f\f\f-->\f\f00:00:01.000\f
text2

vertical tab
\v\v\v00:00:00.000\v\v\v\v-->\v\v00:00:01.000\v
invalid