HTML API: Introduce CSS class-list splitter. #23395

Job	Run time
Gutenberg running from src / MacOS	10m 39s
Core running from src / Linux	2m 18s
Gutenberg running from src / Linux	8m 41s
Gutenberg running from build / MacOS	19m 2s
Core running from build / Linux	2m 40s
Gutenberg running from build / Linux	9m 7s
Gutenberg running from src / Windows	13m 19s
Core running from build / Windows	4m 6s
Gutenberg running from build / Windows	12m 55s
Core running from build / MacOS	3m 16s
Core running from src / Windows	3m 56s
Core running from src / MacOS	2m 58s
Slack Notifications	0s
Failed workflow tasks	0s
	1h 32m 57s

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