css-align-3:
Note: The edges used to synthesize baselines from a box depend on their formatting context: inline-level boxes synthesize from their margin edges [CSS-INLINE-3], table cells synthesize from their content edges [CSS2], and grid and flex items synthesize from their border edges [CSS-GRID-1] [CSS-FLEXBOX-1].

https://drafts.csswg.org/css-align-3/#baseline-export


We are synthesizing the "alignment-baseline" (https://drafts.csswg.org/css-inline-3/#alignment-baseline-property), which has an initial value of "baseline." This defers to the "dominant-baseline" (https://drafts.csswg.org/css-inline-3/#alignment-baseline-property) of the parent. "dominant- baseline," has an initial value of "auto":

Equivalent to alphabetic in horizontal writing modes and in vertical writing modes when text-orientation is sideways. Equivalent to central in vertical writing modes when text-orientation is mixed or upright.
However, in SVG text, the origin point of glyphs (used for coordinate-based glyph positioning) is always handled as for central in vertical writing modes.