https://bugzilla.mozilla.org/show_bug.cgi?id=1293427
1293427 - Intermittent TEST-UNEXPECTED-TIMEOUT | /dom/nodes/Element-firstElementChild-entity.svg |...
RESOLVED (nobody) in Testing - web-platform-tests. Last updated 2016-08-09.
intermittenttestunexpectedtimeout
https://www.w3schools.com/jsref/prop_element_firstelementchild.asp
HTML DOM Element firstElementChild Property
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html domelementproperty