[imsc] font selection rules under-specified? (#516)

nigelmegitt has just created a new issue for https://github.com/w3c/imsc:

== font selection rules under-specified? ==
IMSC 1.2 introduces support for the `#font` feature from TTML2. This feature allows attributes such as font family name, unicode range, style, and weight to be specified, which can be used when selecting an actual font resource to use based on the computed style set that applies to a character or section of text within a span.

When we specified this we discussed the CSS3 `@font-face` rule and I _think_ the working assumption was that we were basing the font selection semantics on that specification. However TTML2 does not explicitly reference that rule anywhere, and certainly not in the section that defines font resources. Further, TTML2 does not define font selection semantics that are coincident with the `@font-face` semantics. The section on [tts:fontSelectionStrategy](https://www.w3.org/TR/ttml2/#style-attribute-fontSelectionStrategy) mentions only:

> When selecting among font resources that satisfy the font selection criteria, font resources that are referenced by a font element have priority over application or system supplied font resources.

without detailing any other algorithm for the defined term "[font selection process]".

I would in the first instance expect TTML2 to describe the semantics for font selection, by referencing `@font-face`. However, given that it does not do so, I think IMSC 1.2 must do so instead.

Reviewing IMSC 1.2 for this, I find:

* [css-fonts-3] is listed in the Normative References section but does not appear to be referenced from the document.
* There is no definition of the semantics for selecting a font given available font resources.

I think this needs to be addressed either in TTML2 or in IMSC 1.2. I'm open to either specification addressing this, but if we decide to address it in TTML2 then we should transfer this issue to the w3c/ttml2 repository.

Please view or discuss this issue at https://github.com/w3c/imsc/issues/516 using your GitHub account

Received on Thursday, 9 January 2020 10:16:17 UTC