So sorry, David, I should have acknowledged receipt of your most helpful suggestion some hours ago, but the moment I opened it something else required my attention and I have only just got back to reading it now. Thank you very much indeed — most helpful.

L. David Baron wrote:

Yes.  See https://drafts.csswg.org/css-conditional/#at-supports and
https://developer.mozilla.org/en-US/docs/Web/CSS/@supports .

-David

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Philip Taylor