[csswg-drafts] [css-inline-3] make `text` of `leading-trim` interoperable? (#3978)

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

== [css-inline-3] make `text` of `leading-trim` interoperable? ==
Unfortunately all impls compute different positions for `text-top`, and that the `text` value using it doesn't make this property interoperable. I would like the property to provide at least one value that is interoperable across platforms (browsers/OS).

I discussed with @litherum on this before. IIUC we would like to have two text-top/bottom metrics; one which is interoperable, another which is compatible with the current layout engine of the platform.

How about something like:

| Value | Description |
----|----
| **_platform-text_** | Use the text-top/bottom metric. This value matches to metric used by the layout, but may vary by platform. |
| **_typo-text_** | Use the text-top/bottom metric using `sTypoAscender`/`sTypoDescender` from [`os/2`](https://docs.microsoft.com/en-us/typography/opentype/spec/os2) table in OpenType fonts, or the tightest metrics on other font systems that can provide multiple text metrics. |
| **_em-height_** | Use the text-top/bottom metrics using the synthesized em-height metrics. The em-height metrics is computed by normalizing the `typo-text` metrics to 1em. For example, the text-top em-height for OpenType is computed by `sTypoAscender / (sTypoAscender + sTypoDescender) * font-size`. |

@fantasai 
The _em-height_ is a metric invented by Gecko, @jfkthame explained [here](https://github.com/whatwg/html/issues/2470#issuecomment-291425136).

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/3978 using your GitHub account

Received on Monday, 27 May 2019 03:31:48 UTC