DNS Prefetch or Preconnect Cross-Domain URLs

It isn't always appropriate to prefetch external (cross-domain) urls, however, some optimizations can still be made for external links by requesting the browser prefetch the dns records or preconnect to the host. Twitter has enabled such a service while hovering over tweets to external urls.

I would like to propose appending dns-prefetch or preconnect html tags when a user hovers or begins to click off-domain url.

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