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Bartek from the Deno team here. No promises yet, but we're looking into feasibility of it.

The docs say it's planned but no proper roadmap for it yet.

No customizable programmable browser runtimes exist for those.

What do you mean by that? Does this[1] not count?

Correct, that isn't a browser it uses a webview. The limited webview version of Deno Desktop could work like that on phones.