debug: Provide error for illegal nesting of <button> and <a> #1487

Job	Run time
Build & Test / Build & Test	1m 12s
Prepare environment	6s
Bench text-update / Bench text-update/text-update	1m 12s
Bench replace1k / Bench table-app/replace1k	2m 43s
Bench create10k / Bench table-app/create10k	2m 28s
Bench filter-list / Bench filter-list/filter-list	1m 28s
Bench many-updates / Bench many-updates/many-updates	1m 55s
Bench todo / Bench todo/todo	1m 10s
Bench 03_update10th1k_x16 / Bench table-app/update10th1k	1m 23s
Bench hydrate1k / Bench table-app/hydrate1k	2m 31s
	16m 8s

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