Re: [ttml2] Ways to make span background height be lineHeight

Dear all,

I think this issue and the discussion documented here:

https://github.com/w3c/ttml2/issues/150

bring up a lot of important questions that are not only of relevance for 
TTML 2 but also for TTML 1 and all other specs that take TTML 1 as a base.

I think that it is important that we give this issue enough room for 
discussion in our meetings and on the reflector.

Best regards,

Andreas

Am 03.03.2016 um 16:04 schrieb Nigel Megitt via GitHub:
> nigelmegitt has just created a new issue for
> https://github.com/w3c/ttml2:
>
> == Ways to make span background height be lineHeight ==
> Many (most?) CSS implementations draw the background box behind inline
>   areas with a height less than the line height, meaning that
> successive lines are drawn with gaps between their background areas,
> if there's no block level background color set. We want to be able to
> avoid that situation, since it negatively impacts readability and
> hence accessibility when background colors are applied to spans.
>
> See also the discussions minuted
> [here](https://www.w3.org/2016/02/18-tt-minutes.html#item05) and
> [here](https://www.w3.org/2016/02/25-tt-minutes.html#item05) and [this
>   
> thread](https://lists.w3.org/Archives/Public/public-tt/2016Feb/0104.html).
>
> ## First question: is the height actually defined?
>
> [CSS 2
> §10.6.1](https://www.w3.org/TR/CSS2/visudet.html#inline-non-replaced):
>
>> The height of the content area should be based on the font, but this
>   specification does not specify how. A UA may, e.g., use the em-box or
>   the maximum ascender and descender of the font. (The latter would
> ensure that glyphs with parts above or below the em-box still fall
> within the content area, but leads to differently sized boxes for
> different fonts; the former would ensure authors can control
> background styling relative to the 'line-height', but leads to glyphs
> painting outside their content area.)
>> Note: level 3 of CSS will probably include a property to select which
>   measure of the font is used for the content height.
>
> See also §10.6.6 and §10.6.7 in that specification.
>
> I'm still trying to work out what this means and whether it provides
> flexibility to implementers to make the inline box height be the same
> as the line height, or if there's some other calculation that applies.
>
> ## Second question: do we need syntax and semantics for clarifying it
> in TTML2?
>
> Pending the answer to the first question.
>
> Please view or discuss this issue at
> https://github.com/w3c/ttml2/issues/150 using your GitHub account
>


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Received on Thursday, 10 March 2016 14:55:58 UTC