[css-device-adap] Encourage both mobile and desktop implementations

Author: frivoal

css-device-adapt/Overview.bs

@@ -129,6 +129,20 @@ behaviour per default (actual viewport is initial viewport).</p>
 <h2 id="atviewport-rule">
 The <dfn at-rule>@viewport</dfn> rule</h2>
 
+<div class=note>
+<strong class="advisement">UA vendors implementing this specification
+are strongly encouraged to do so both for their mobile and desktop browsers.
+The ''@viewport'' mechanism is designed to be usable and useful
+on all browsers, not only mobile ones.
+However, if support is only available on mobile browsers for a significant time,
+there is a risk that authors would write ''@viewport'' rules that work on mobile
+but do the wrong if applied by a destkop browser.
+This would make it difficult to later add support ''@viewport'' in desktop browsers.</strong>
+<p>An example of such misguided use would be to write <code highlight="css">@viewport { width: 320px; }</code>
+instead of <code highlight="css">@viewport { width: auto; }</code>
+to make a document “mobile friendly”.
+</div>
+
 The ''@viewport'' <a href="http://www.w3.org/TR/CSS21/syndata.html#at-rules">at-rule</a>
 consists of the @-keyword followed by a block of descriptors
 describing the viewport.