Add viewport segment media queries (#6234)

Author: dlibby-

mediaqueries-5/Overview.bs

@@ -1252,6 +1252,46 @@ Inline-Axis Overflow: the '@media/overflow-inline' feature</h3>
 	and the very concept doesn't seem to make much sense,
 	so there is intentionally no ''paged'' value for '@media/overflow-inline'.
 
+<h3 id='mf-horizontal-viewport-segments'>
+Horizontal Viewport Segments: the '@media/horizontal-viewport-segments' feature</h3>
+
+	<pre class="descdef mq">
+	Name: horizontal-viewport-segments
+	Value: <<integer>>
+	For: @media
+	Type: range
+	</pre>
+
+	The '@media/horizontal-viewport-segments' media feature describes the number of logical segments of
+    the viewport in the horizontal direction. 
+
+    The '@media/horizontal-viewport-segments' media feature is <a>false in the negative range</a>.
+
+    When the viewport is split by one or more hardware features (such as a fold or a hinge between
+    separate displays) that act as a logical divider, segments are the regions of the viewport that
+    can be treated as logically distinct by the page.
+
+<h3 id='mf-vertical-viewport-segments'>
+Vertical Viewport Segments: the '@media/vertical-viewport-segments' feature</h3>
+
+	<pre class="descdef mq">
+	Name: vertical-viewport-segments
+	Value: <<integer>>
+	For: @media
+	Type: range
+	</pre>
+
+	The '@media/vertical-viewport-segments' media feature describes the number of logical segments of
+    the viewport in the vertical direction.
+
+    The '@media/vertical-viewport-segments' media feature is <a>false in the negative range</a>.
+
+	<div class='example'>
+		This media query detects a viewport that has exactly two segments that are side-by-side.
+
+		<pre>@media (horizontal-viewport-segments: 2) and (vertical-viewport-segments: 1) { … }</pre>
+	</div>
+
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