[css-box] document tree -> element tree

Author: fantasai

css-box-3/Overview.bs

@@ -28,14 +28,14 @@ Introduction {#intro}
 
 	<p>CSS describes how each element
 	and each string of text in a source document
-	is laid out by transforming the <a>document tree</a>
+	is laid out by transforming the <a>element tree</a>
 	into a set of <a>boxes</a>,
 	whose size, position, and stacking level on the <a>canvas</a>
 	depend on the values of their CSS properties.
 
 	Note: <a href="https://www.w3.org/TR/css-cascade/">CSS Cascading and Inheritance</a>
 	describes how properties are assigned to elements in the box tree,
-	while [[css-display-3#intro]] describes how the <a>document tree</a>
+	while [[css-display-3#intro]] describes how the <a>element tree</a>
 	is transformed into the <a>box tree</a>.
 
 	<p>Each CSS <a>box</a>

css-box-4/Overview.bs

@@ -24,14 +24,14 @@ Introduction {#intro}
 
 	<p>CSS describes how each element
 	and each string of text in a source document
-	is laid out by transforming the <a>document tree</a>
+	is laid out by transforming the <a>element tree</a>
 	into a set of <a>boxes</a>,
 	whose size, position, and stacking level on the <a>canvas</a>
 	depend on the values of their CSS properties.
 
 	Note: <a href="https://www.w3.org/TR/css-cascade/">CSS Cascading and Inheritance</a>
 	describes how properties are assigned to elements in the box tree,
-	while [[css-display-3#intro]] describes how the <a>document tree</a>
+	while [[css-display-3#intro]] describes how the <a>element tree</a>
 	is transformed into the <a>box tree</a>.
 
 	<p>Each CSS <a>box</a>