[csswg-drafts] [css-inline-3] Propose N/A value for em-top and em-bottom baselines (#5380)

kojiishi has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-inline-3] Propose N/A value for em-top and em-bottom baselines ==
A follow up for #5312.

I'd like to propose using `undefined` in JavaScript for when the font doesn't have none of top, bottom, nor central of em-box.

Imagine a case:
```javascript
const ctx = canvas.getContext('2d');
ctx.font = '20px Arial, CJKFont';
const metrics = ctx.measureText('日本語');
```
I think the expectation is to get em-box from the `CJKFont`.

Another example:
```javascript
ctx.font = '20px Arial, CJKFont1, CJKFont2';
```
and `CJKFont1` doesn't have BASE table but `CJKFont2` has. Is the expectation to synthesize from `Arial`, synthesize from `CJKFont1`, or use BASE table from `CJKFont2`?

Because this is JavaScript API, if author knows that the first available font doesn't have accurate metrics, they can choose options such as synthesize from ascent/descent of the first available font, fallback to the next font, or something else. If browser returns possibly inaccurate synthesized values, authors doesn't have a good way to determine how much they can trust it.

WDYT?

/cc @jfkthame @litherum @fantasai @annevk @yiyix @macnmm

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5380 using your GitHub account


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Received on Friday, 31 July 2020 23:10:18 UTC