Do not restart CSS animations for identical elements

I've got a spinner, and the section of the page with the spinner is replaced periodically. When this happens, the spinner's CSS animation restarts. I want it to continue for as long as an identical spinner is returned.

The issue names no files, tests, or entry points. Start by reproducing the spinner during periodic section replacement and trace htmx's DOM replacement behavior; done means an identical spinner continues its CSS animation while a changed spinner can start anew.

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