Can Flexbox be used in Canvas Catalog Custom Certificates?

[dacarter]

I've tried to get Canvas Catalog Custom Certificates to render Flexbox elements, but it just seems to place those elements into a standard block context.  Has anyone successfully implemented a flexbox design for Custom Certs?

[dacarter]

Having discovered that there is such a thing as a "Canvas HTML Whitelist", AKA Canvas HTML Editor Allowlist, about which I knew nothing, I am replying to myself just to note it, and so that others might stumble upon it here, if it was missed elsewhere.