V3: activating accessibility results in "Math input error"

Created by: NSoiffer

Visit the page https://mathjax.github.io/MathJax-demos-web/mml-chtml.html. It renders well. Right click on some math, choose Accessibility:Activate. The math is now replaced by "Math input error". Here's a small snippit:

The page's heading says "...v3 beta...", but it appears to link to the V3 release. Here's the script tags from the page:

<script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
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  | <script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/mml-chtml.js"></script>