The previous implementation centered initials by measuring `<text>`
with `getBBox()`, which returns the font's advance-width by
ascent-to-descent band, not the ink of the glyphs. On fonts with an
asymmetric band, initials rendered off-center. Marianne is a
particularly clear case: ascent 1131 / descent 256 puts the band
center 87.5/1000 above the caps' optical center, so every avatar
sat ~4.6 viewBox units (~1.5–1.8 px) too low. A follow-up rewrite
using canvas `actualBoundingBox*` metrics fixed it but pulled in a
runtime measurement rig (shared canvas, ref, state, layout effect,
`fonts.load` + `loadingdone`, plus combining-mark stripping) just
to place two uppercase letters.
Since initials are always uppercased, optical centering has a
closed form: `baseline = center + capHeight/2`. Real-world text
fonts have cap heights in a narrow ~0.66–0.73 em band (Marianne is
0.70), so:
translateY(calc(var(--avatar-cap-height, 0.7) * 0.5em))
is exact for the stock font and within ~0.4 px for any plausible
replacement. No JS, SSR-safe, no first-paint jump, no font-loading
race. Accents float above the cap box instead of dragging the
letter down.
The single font-dependent number remaining (cap height) is exposed
as a CSS variable, so self-hosters overriding the font can override
it next to the font itself, or leave the default. Once
`text-box: trim-both cap alphabetic` ships broadly, even the
variable can go.
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lebaudantoine