Clarify that border-radius values cannot be negative. <http://lists.w3.org/Archives/Public/www-style/2012Mar/0258.html>

Author: fantasai

css3-background/Overview.html

@@ -2759,7 +2759,7 @@ <h3 id=the-border-radius><span class=secno>5.1. </span>Curve Radii: the
    copied from the first. If either length is zero, the corner is square, not
    rounded. Percentages for the horizontal radius refer to the width of the
    border box, whereas percentages for the vertical radius refer to the
-   height of the border box.
+   height of the border box. Negative values are not allowed.
 
   <div class=figure>
    <p><img alt="Diagram of the inscribed ellipse" src=corner.png>

css3-background/Overview.src.html

@@ -1917,7 +1917,7 @@ <h3 id="the-border-radius">Curve Radii: the 'border-radius' properties</h3>
 If either length is zero, the corner is square, not rounded.
 Percentages for the horizontal radius refer to the width of the
 border box, whereas percentages for the vertical radius refer to
-the height of the border box.
+the height of the border box. Negative values are not allowed.
 
 <div class="figure">
   <p><img alt="Diagram of the inscribed ellipse" src="corner.png">