The descendants of a 07001 must not generate Boxes in the
formatting tree. Instead,the contents of the 07002
generate Boxes as if they were the contents of the element instead.

CSS Pseudo-Elements描述:: before和:: after as

these pseudo-elements generate Boxes as if they were immediate
children of their originating element

那么哪些是真的？

>首先，shadow主机的所有内容(不包括:: before和:: after)由活动阴影树的内容替换。然后，:: before和:: after生成阴影主机中的框。
>首先，:: before和:: after在shadow主机中生成框。然后，影子主机的所有内容(包括:: before和:: after)由活动阴影树的内容替换。

Firefox和Chrome做的前者，但规范描述的行为？

var root = document.querySelector('div').createShadowRoot();
root.innerHTML = "<p>Shadow content</p>";

div::before,div::after {
  content: 'Generated content';
}

<div>Content</div>