rejection of scriptlevel attribute unexpected

[jhursty]

I'm trying to use scriptlevel to bypass the normal reduction in size applied to arguments to mfrac. E.g.:

<div class="formula" id="eq-a-2">
  <math>
    <mrow>
      <ms>sinc</ms><mo>&ApplyFunction;</mo>
      <mo>(</mo>
      <mi>x</mi>
      <mo>)</mo>
    </mrow>
    <mo>=</mo>
    <mfrac scriptlevel="-1">
      <mrow>
        <ms>sin</ms><mo>&ApplyFunction;</mo>
        <mo>(</mo>
        <mi>x</mi>
        <mo>)</mo>
      </mrow>
      <mi>x</mi>
    </mfrac>
  </math>
</div>

Chrome responds to this markup by sizing the mfrac contents the same as everything up to '=', which is the layout I want.

The C.I. responds with the error message:
Attribute "scriptlevel" not allowed on element "mfrac" at this point.

for the line:
<mfrac scriptlevel="-1">

Run node ${GITHUB_ACTION_PATH}/../scripts/build.mjs deploy
  node ${GITHUB_ACTION_PATH}/../scripts/build.mjs deploy
  cat ./build/vars.txt > "$GITHUB_OUTPUT"
  shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
  env:
    AWS_REGION: us-east-1
    AWS_S3_BUCKET: html-doc-pub
    AWS_ROLE: arn:aws:iam::***:role/gh-actions-html-pub
    CANONICAL_LINK_PREFIX: https://doc.smpte-doc.org/
    REPOSITORY_NAME: st2067-101-private
    AWS_DEFAULT_REGION: us-east-1
    AWS_ACCESS_KEY_ID: ***
    AWS_SECRET_ACCESS_KEY: ***
    AWS_SESSION_TOKEN: ***
    AWS_S3_REGION: us-east-1
    AWS_S3_KEY_PREFIX: st2067-101-private/
Current branch: main
Current commit hash: f507126a019824bcde997cf45b45daf5c3ff56b7
"file:/home/runner/work/st2067-101-private/st2067-101-private/doc/main.html":2151.5-2151.28: error: Attribute "scriptlevel" not allowed on element "mfrac" at this point.