Disabled SVG should fall back to other image formats

At https://www.bamsoftware.com/papers/fronting/, the figures don't show up with the security slider on the "High" setting in Tor Browser 6.5a6. Instead, it just shows the alt text:

The HTML is using the picture element to allow fallback from SVG to PNG:

<picture>
<source srcset=fronting.svg type="image/svg+xml">
<img src=fronting.png alt="A schematic diagram...">
</picture>

It would be nice if the way Tor Browser disabled SVG was compatible with this usage, to allow the browser to try PNG next.

On the other hand, I see the MDN page says these API are experimental, so go ahead and close this if it seems esoteric or unsupported.

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