[css-size-adjust] text-size-adjust is non-negative

Like other sizing properties, text-size-adjust does not
accept negative values.

Author: ericwilligers

css-size-adjust-1/Overview.bs

@@ -66,7 +66,7 @@ Module interactions {#placement}
 Values {#values}
 ----------------
 
-  This specification follows the <a href="https://www.w3.org/TR/CSS21/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]].
+  This specification follows the <a href="https://www.w3.org/TR/CSS21/about.html#property-defs">CSS property definition conventions</a> from [[!CSS21]].
   Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
   Other CSS modules may expand the definitions of these value types.
 
@@ -168,7 +168,8 @@ Size adjustment control: the 'text-size-adjust' property {#adjustment-control}
       <dd>Renderers must not do size adjustment when displaying on a small device.</dd>
     <dt><dfn>&lt;percentage&gt;</dfn></dt>
       <dd>When displaying on a small device, renderers must not do size adjustment but instead the computed value of 'font-size' must be multiplied by this percentage.
-      <p class="note">Note: This means that 'text-size-adjust: 100%;' is equivalent to 'text-size-adjust: none;'.</p></dd>
+      <p class="note">Note: This means that 'text-size-adjust: 100%;' is equivalent to 'text-size-adjust: none;'.</p>
+      <p>Negative values are not allowed.</p></dd>
   </dl>
 
   Issue: "Small device" is not well defined. We should clearly specify