XSL:FO, css-inline-3 2002 WD, and SVG1.1 define the central baseline in terms of the ascent / descent metrics. However, we know that these metrics are not interoperable. We could define it as the same mysterious metric used for vertical-align: text-top and vertical-align: text-bottom (which is how I've currently drafted the definition), but to the extent that ends up being equivalent to this non-interoperable ascent/descent metrics (does it?), we have the same problem...

The central baseline is defined as the default for vertical text layout. And that default really needs to be something interoperable. So I see two ways forward here:

• Option A: Define central in a way that everyone can implement.
• Option B: Add an ideographic-central baseline halfway between the ideo and idtp baselines, which falls back to the ambiguously-defined central baseline when those metrics are missing, and make that the default in vertical writing modes.

Interested to hear from @litherum, @jfkthame, @kojiishi, @macnmm ...