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Re: [csswg-drafts] [css-inline-3] About the central baseline (#5177)

From: Myles C. Maxfield via GitHub <sysbot+gh@w3.org>
Date: Sat, 29 Aug 2020 09:01:46 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-683261341-1598691704-sysbot+gh@w3.org>
tl;dr: the resolution is good, let’s proceed.

Yeah, I should have been more clear. Sorry about that.

A reasonable path forward here is to say “if your text library doesn’t expose the metrics, the CSS engine has to synthesize them instead.” And, since we can synthesize whatever values we want, we can synthesize the values that happen to cause no behavior change.

That being said, I do think there’s a reasonable argument to be made that the behavior change proposed in this thread would be a progression, rather than a regression, so the statement I made about compat isn’t particularly relevant. It’s likely that halfway between the ideographic top and ideographic bottom will be better than halfway between the ascent and descent, and it’s likely that ideographic central will be better yet.

(Of course, all this is modulo the engine’s freedom to ignore/fix-up any data we see in the font that doesn’t look reasonable. So even if the baselines are present, if they’re crazy we can just ignore them, or just pretend they are set to something better. We do this already for existing metrics, and these new ones should be no different. Philosophically, it’s the engine’s job to make things look good, regardless of whether or not the data in the fonts is bogus/missing or not.)

Therefore, that was a long way of saying that we should move forward with this resolution, with the above caveats. I was really just stating my concerns for the record; not actually objecting or resisting.

(Aside: In Core Text, the baselines in the font are used/observable if you use the CT line-layout facilities which automatically handle this baseline stuff. WebKit can’t use that API though, since CSS defines its own line layout algorithms. So we instead use the lower-level API, which doesn’t happen to expose a getter for this stuff. The Core Text team is aware of and sympathetic to CSS’s needs here.)

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