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Without regions the default font is '5vh sans-serif'. With regions font is '(0.0533/1.3)vh sans-serif' and line-height is '0.0533vh'. (The values are 100 times too small, but that's just a typo.) This is weird, as mixing regions and plain cues would result in differently sized text unless overridden. Those who want to emulate 608 (80/15=5.33) can do so by changing the font using CSS, e.g. ::cue { font: 4.1vh monospace; line-height: 5.33vh; }
Fair enough. Happy to drop the font fix and leave that to the emulation spec. The line-height fix was done such that lines in rollup mode don't change height based on the text within. See also Rick's helpful article at http://rickeyre.ca/2014/04/15/number-of-lines-of-text.html .
Rick, are you using the average line height in Firefox's WebVTT implementation? The spec says "Let step be the height of the first line box in boxes." and that's what Presto does at least. I'm not exactly sure what it is that Blink/WebKit does...
Setting line-height to some value doesn't help with the height depending on the text. I think the line height only changes if you change the font or font size, which is still possible. So I don't see any reason to specify a line-height that is different from the CSS initial value here.
https://github.com/w3c/webvtt/pull/242