Found a new reason to use elem.classList today.

Had a demo that was looping over a number of elements setting elem.className = 'box future' or elem.className = 'box'. Looking in the new DevTools Timeline Frame mode, it was already busting at the seams with the 16.6ms of each frame mostly already accounted for.

See in the video how I used DevTools to identify how to end up with better performance:
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