[csswg-drafts] [css-display] Clarify element tree definition

Loirooriol has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-display] Clarify element tree definition ==
[CSS Display](https://drafts.csswg.org/css-display-3/#intro) says

> CSS takes a source document, organized as a tree of elements and text nodes, and renders it onto a canvas

This is confusing because the source document is organized as a [DOM tree](https://dom.spec.whatwg.org/#trees) of [DOM nodes](https://dom.spec.whatwg.org/#concept-node). Not just elements and text nodes.

As far as I know, the CSS element tree is derived from the DOM tree as follows:

 - The root is the document element.
 - All non-element and non-text nodes are removed.
 - A `::before` pseudo-element is inserted as the first child of each element node.
 - An `::after` pseudo-element is inserted as the last child of each element node.
 - A `::marker` pseudo-element is inserted at some places (to be exactly determined in #1793).

All of this should be properly defined in the spec.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/1810 using your GitHub account