Re: [css-fonts] Conflicting tags in font-feature-settings

On 16/3/16 00:12, Xidorn Quan wrote:
> On Wed, Mar 16, 2016 at 7:59 AM, Myles C. Maxfield <mmaxfield@apple.com
> <mailto:mmaxfield@apple.com>> wrote:
>
>
>     > On Mar 14, 2016, at 7:03 PM, Xidorn Quan <quanxunzhen@gmail.com <mailto:quanxunzhen@gmail.com>> wrote:
>     >
>     > Hi,
>     >
>     > It seems to me that the spec is not clear about what should happen if conflicting tags are specified for font-feature-settings. For example, if you set "font-feature-settings: 'hwid', 'twid', 'qwid'", only one of them can be chosen, but it is not clear which would win in that case.
>
>     Is there a repository somewhere listing every combination of
>     conflicting font features?
>
>
> I believe at least most of variants conflict with each other. Not sure
> about other features.
>

IMO, this is out of scope for CSS-Fonts. The font-feature-settings 
property simply specifies some OpenType feature tags to be set.

What happens from there on -- and in particular, how "competing" 
features interact -- is a matter for the OpenType spec,[1] and in most 
cases (except where script-specific shaping specifications mandate a 
particular order of feature application) it's the font designer who 
determines the interaction or relative priority of the features, based 
on the ordering of lookups within the font.

So for example, if the style calls for

   font-feature-settings: 'hwid', 'twid', 'qwid';

all of these feature tags will be set; which glyphs will actually result 
is dependent on how the font designer orders the relevant lookups within 
the font.

FWIW, there cannot be any definitive "listing [of] every combination of 
conflicting font features", because so much of the behavior (and 
interaction) of features is under the control of the font designer. Some 
of the feature descriptions in the registry[2] do mention interactions 
with other features, but this can only be considered informative, not 
exhaustive. (And font developers may also implement novel features that 
do not appear in any official registry.)

JK

[1] https://www.microsoft.com/typography/otspec/
[2] https://www.microsoft.com/typography/otspec/featuretags.htm

Received on Wednesday, 16 March 2016 12:02:58 UTC