Add 'prefers-motion' media feature

Implements Issue: w3c#442
Alternative to w3c#586

Having a good wording of the feature also gets around default value evaluating to true instead of false. Ref: w3c#586 (comment)

mediaqueries/Overview.bs

@@ -1806,6 +1806,53 @@ All Available Interaction Capabilities: the 'any-pointer' and 'any-hover' featur
 		or to hide them altogether to reduce visual clutter if ''(any-pointer: fine)'' is false.
 	</div>
 
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+
+<h2 id='mf-user-preferences'>
+User Preference Media Features</h2>
+
+<h3 id="prefers-motion">
+Detecting the preference for less or no motion on the page: the 'prefers-motion' feature</h3>
+
+	<pre class='descdef mq'>
+	Name: prefers-motion
+	Value: default | reduced | disabled
+	For: @media
+	Type: discrete
+	</pre>
+
+	The 'prefers-motion' media feature is used to detect if the user
+	has requested the system minimize or disable the amount of animation
+	or motion it uses.
+
+	<dl dfn-type=value dfn-for="@media/prefers-motion">
+		<dt><dfn>default</dfn>
+		<dd>
+			Indicates that the user has made no preference known
+			to the system. This keyword value evaluates as true
+			in the <a>boolean context</a>.
+
+		<dt><dfn>reduced</dfn>
+		<dd>
+			Indicates that user has notified the system that they
+			prefer an interface that minimizes the amount of movement
+			or animation, preferably to the point where all non-essential
+			movement is removed.
+		<dt><dfn>disabled</dfn>
+		<dd>
+			Indicates that user has notified the system that they
+			prefer an interface that disables movement or animation,
+			preferably to the point where all movement is removed.
+	</dl>
+
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