[css-position-3] Rewrite intro

Author: tabatkins

css-position-3/Overview.bs

@@ -103,30 +103,33 @@ Introduction</h2>
 
 	<em>This section is not normative.</em>
 
-	In CSS, a box can be laid out according to three positioning schemes:
-
-	<dl>
-		<dt>normal flow
-		<dd>
-			In CSS, [=normal flow=] includes
-			<a href="https://www.w3.org/TR/CSS2/visuren.html#block-formatting">block formatting</a> of block-level boxes,
-			<a href="https://www.w3.org/TR/CSS2/visuren.html#inline-formatting">inline formatting</a> of inline-level boxes,
-			and <a lt="relative positioning" spec="css2">relative</a> and <a lt="sticky positioning">sticky</a> positioning of block-level and inline-level boxes.
-
-		<dt>floats
-		<dd>
-			In the <a>float</a> model,
-			a box is first laid out according to the <a>normal flow</a>,
-			then taken out of the flow and positioned, typically to the left or right.
-			Content may flow along the side of a [=float=].
-
-		<dt>absolute positioning
-		<dd>
-			In the absolute positioning model,
-			a box is removed from the [=normal flow=] entirely
-			(it has no impact on later siblings)
-			and assigned a position with respect to a [=containing block=].
-	</dl>
+	The CSS layout algorithms, by default,
+	size and position boxes in relation to each other
+	so that nothing overlaps.
+
+	This specification defines several ways to violate these assumptions when needed,
+	moving elements around in ways that can make them overlap other content:
+
+	* [=Relative positioning=], which visually shifts a box relative to its laid-out location.
+	* [=Sticky positioning=], which visually shifts a box relative to its laid-out location
+		in order to keep it visible when a scrollable ancestor
+		would otherwise scroll it out of sight.
+	* [=Absolute positioning=], which ignores normal layout entirely, 
+		pulling the element out of flow and
+		and positioning it relative to its [=containing block=]
+		with no regard for other content.
+	* [=Fixed positioning=], which absolutely positions the box
+		and affixes it to the viewport or page frame
+		so that it is always visible.
+
+	These [=positioning schemes=],
+	controlled by the 'position' property and the [=inset properties=],
+	are powerful
+	but easy to misuse.
+	With appropriate care,
+	they allow many interesting and useful layouts
+	that couldn't otherwise be achieved with standard layout rules;
+	without, they allow a page to be laid out in an unusable overlapping jumble of content.
 
 <h3 id="placement">
 Module Interactions</h3>