The aim here is to center text vertically (with cap height understood as the height of the text).
Each text should be in the middle of an ellipse.

The red text uses dominant-baseline="central", which is supposed to work, no matter the typeface.
It does work in the browser — but not on Commons. Instead, the baseline is centered.

The green text is initially placed with the baseline at the target height, and then moved down with a transform.
That is unreliable, because it depends on the typeface.

<g font-size="15" style="font-family: Arial, Helvetica, sans-serif;">

    <text x="25" y="20" text-anchor="middle" dominant-baseline="central" style="fill: red;">678</text>

    <text x="25" y="50" text-anchor="middle" transform="translate(0, 5.6)" style="fill: green;">678</text>

</g>