Interpretation of spaceAfterScript for mmultiscripts's prescripts

I stumbled on some interop issue while exporting tests from Gecko.

From https://learn.microsoft.com/en-us/typography/opentype/spec/math:

spaceAfterScript Extra white space to be added after each subscript and superscript. Suggested: 0.5 pt for a 12 pt font. (Note that, in some math layout implementations, a constant value, such as 0.5 pt, may be used for all text sizes. Some implementations may use a constant ratio of text size, such as 1/24 of em.)

MathML Core and Chromium places the space before each prescript at https://w3c.github.io/mathml-core/#base-with-prescripts-and-postscripts:

For each subscript/superscript pair of mmultiscripts prescripts:
Increment inline-offset by SpaceAfterScript.
...

But Firefox seems to do it the other way around, i.e. place the space after each prescript.

So the former option makes the rendering more symmetric with respect to the base, but the latter makes it more consistent with the parameter name.

Since we have space between scripts anyway, the significant difference is that the former adds a space between the leftmost scripts (assuming LTR) and the content before ; and the latter adds a space between the base and its closest prescripts.

Read the linked OpenType Math specification and MathML Core sections on SpaceAfterScript and mmultiscripts prescripts. Compare the described Firefox and Chromium behaviors, then determine which placement the specification intends. Done means the ambiguity and expected rendering behavior are resolved in the issue or specification.