[css-ui-3] Relax from MUST to SHOULD constraints on negative outline-offset

Resolution: https://logs.csswg.org/irc.w3.org/css/2017-01-12/#e760940

Author: frivoal

css-ui/Overview.bs

@@ -36,7 +36,6 @@ Ignored Terms: outline-left
 Ignored Terms: ime-mode
 At Risk: The <a href="#overflow-string">&lt;string&gt;</a> value of the 'text-overflow' property
 At Risk: The 2-value syntax of the 'text-overflow' property
-At Risk: Constraints on <a href="#negative-offset">negative values</a> of the 'outline-offset' property being expressed with MUST rather than with SHOULD
 </pre>
 <pre class=link-defaults>
 spec:css-backgrounds-3; type:property; text:border-width
@@ -488,18 +487,15 @@ the following rule can be used:
 <p id=negative-offset>Negative values must cause the outline
 to shrink into the border box.
 Both the height and the width of outside of the shape
-drawn by the outline must not become smaller
-than twice the computed value of the outline-width property,
+drawn by the outline should not become smaller
+than twice the computed value of the 'outline-width' property,
 to make sure that an outline can be rendered
 even with large negative values.
-User Agents must apply this constraint
+User Agents should apply this constraint
 independently in each dimension.
 If the outline is drawn as multiple disconnected shapes,
 this constraint applies to each shape separately.
 
-Note: Negative value constraints are an at risk feature due to
-<a href="https://lists.w3.org/Archives/Public/www-style/2014Dec/0033.html">interoperability problems</a>, and thus might be dropped from a must to a should.
-
 
 <h2 id="resizing-and-overflow">Resizing &amp; Overflow</h2>
 
@@ -1538,6 +1534,8 @@ This appendix describes changes from the
 	    so that it can also influence things like the navigation caret.
 	<li>Clarify by reference to [[CSS-COLOR-4]] how computed values are handled for caret-color.
 	<li>Mark 'caret-color' as no longer at risk, since there are two implementations.
+	<li>Constraints on the shape of the outline when <a href="#negative-offset">negative values</a> are used for the 'outline-offset' property
+	    where changed from being expressed with MUST to SHOULD.
 </ul>
 
 <h2 class="no-num" id="security-privacy-considerations">Appendix C. Considerations for Security and Privacy</h2>