When innerHTML is set on a mock element, textContent now updates to match (with HTML tags stripped). Many HS tests do `put "foo" into me` (which sets innerHTML) then check textContent. Previously textContent stayed empty because only innerHTML was updated. Also fixes innerHTML="" to fully detach children from parent. 393 → 408/831 HS tests (+15). Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>