[css-display-3] Improve intro explanation of element trees. w3c#4663

Author: fantasai

css-display-3/Overview.bs

@@ -70,17 +70,18 @@ Introduction</h2>
 
 	<p><em>This section is normative.</em>
 
-	CSS takes a source document, organized as
+	CSS takes a source document organized as
 	a <dfn id="element-tree" oldids="css-element-tree" export for=CSS lt="element tree">tree</dfn>
 	of <dfn id="elements" oldids="css-element" export for=CSS lt="element">elements</dfn>
-	and <dfn id="text-nodes" oldids="css-text-node" export for=CSS lt="text node">text nodes</dfn>,
+	(which can contain a mix of other [=elements=] and [=text nodes=])
+	and <dfn id="text-nodes" oldids="css-text-node" export for=CSS lt="text node">text nodes</dfn>
+	(which can contain text),
 	and renders it onto a <a href="https://www.w3.org/TR/CSS2/intro.html#canvas">canvas</a>
 	such as your screen, a piece of paper, or an audio stream.
-	(Some source documents start from more complex trees,
-	such as the DOM,
-	which can have comment nodes
-	and other types of things.
-	[[DOM]]
+	Although any such source document can be rendered with CSS,
+	the most commonly used type is the the DOM. [[DOM]]
+	(Some of these more complex tree types might have additional types of nodes,
+	such as the comment nodes in the DOM.
 	For the purposes of CSS,
 	all of these additional types of nodes are ignored,
 	as if they didn't exist.)