Wide Review Comment 2017: default font size and line heights

[nigelmegitt]

Copy/paste from https://lists.w3.org/Archives/Public/public-tt/2017Sep/0080.html - raising as an issue for tracking/disposition purposes.

The default font size is 5vh, however the default line spacing for addressing line positions is only 5.33vh, representing a line spacing of 106.6% - that would represent an unusually small line height and may result in strange effects. I cannot see anywhere that overrides the line position height based on the computed value of the line-height property.

[silviapfeiffer]

vh stands for viewport height. The line height of 5.33vh stems from CEA-608/708 which provide for 15 lines of content on 80% height, so that gives each line 5.33% height.

NOTE: 5.33vh is only used for regions, not for cues, where line-height is simply the CSS default "normal", which is typically about 1.2% larger.

So we just need to fix this for regions.
I did some forensics on how this happened.

1/ Originally, for regions, line height was fixed to 5.33vh and font-size to 5.33/1.3vh giving sufficient space. #242 removed that because of https://www.w3.org/Bugs/Public/show_bug.cgi?id=25636 .
2/ We also still have #269 .

Now, how to move forward?
I'd rather not change the 5vh default font-size on cues because it's already implemented everywhere and seems to work quite well.
I think we could just replace the 5.33vh with "normal" or with 6vh (which is 5*1.2) to provide more space. That means we ignore the inspiration from CEA608/708, which is fine - if somebody needs it fixed to what CEA608/708 do, they can manually set the font-size and line-height through CSS in STYLE, which is what they have to do already.

Would 6vh work for you?

[nigelmegitt]

OK, changing the default line height seems reasonable - I like the idea of clearly specified values so 6vh seems good, but I'm also aware that "normal" may give better results in terms of readability, so I don't have a strong position there.

Is there a place in the rendering algorithm where the line spacing for addressing line positions is obtained from the applicable style if set? I could not see any steps that adjusted lineHeight from the initial 5.33vh value. Indeed it only appears in §6.1 subsection 12.1 bullet 2. So there appears to be no opportunity to change lines based on an actual value.

[silviapfeiffer]

We have a fixed line height for regions so we're able to calculate the region's height. lineHeight is adjusted when applying the terms of the CSS.

I'll set the line height for regions to 6vh.

We're tracking down how lineHeight converts to the calculation of the region's height in #377 .