Created attachment 346427 [details] A page with custom elements that log when their constructor and connectedCallback are called. When a group of custom elements are created through the parser (either because they are in the main document's HTML or are in a string being set to a connected element's `innerHTML` property), they are all constructed before any have their `connectedCallback` called. For example, for this tree: <c-e id="A"> <c-e id="B"></c-e> </c-e> <c-e id="C"></c-e> Safari calls the constructor and connectedCallback of each in this order: - constructor A - constructor B - constructor C - connectedCallback A - connectedCallback B - connectedCallback C However, the order should be this: - constructor A - connectedCallback A - constructor B - connectedCallback B - constructor C - connectedCallback C Relevant spec text: https://html.spec.whatwg.org/multipage/parsing.html#create-an-element-for-the-token The 'in-document HTML' case would have 'will execute script' set to true, meaning that the interleaved order above is the expected order, AFAICT. I think Safari's `innerHTML` is correct here though: it would have 'will execute script' set to false, so the connectedCallback reactions would all be delayed until they're popped at the end.