[css-backgrounds-3] Handle negative spread radius of inner shadows. w3c#1900 (comment)

Author: fantasai

css-backgrounds-3/Overview.bs

@@ -1995,6 +1995,8 @@ the margin is multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
 where <var>r</var> is the ratio of the border radius to the margin,
 in calculating the corner radii of the margin box shape.
 
+
+
 <h3 id="corner-clipping">Corner Clipping</h3>
 
 <p>Although <a href="#the-border-image">border images</a> are not affected
@@ -2921,8 +2923,10 @@ Shadow Shape, Spread, and Knockout</h4>
   the <i>spread distance</i> (and flooring at zero).
   However, in order to create a sharper corner when the border radius is small
   (and thus ensure continuity between round and sharp corners),
-  when the <i>border radius</i> is less than the <i>spread distance</i>,
-  the <i>spread distance</i> of an outer shadow
+  when the <i>border radius</i> is less than the <i>spread distance</i>
+  (or in the case of an inner shadow,
+  less than the absolute value of a negative <i>spread distance</i>),
+  the <i>spread distance</i>
   is multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
   where <var>r</var> is the ratio of the border radius to the <i>spread distance</i>,
   in calculating the corner radii of the spread shadow shape.
@@ -3057,8 +3061,8 @@ Changes since the 17 October 2017 Candidate Recommendation</h3>
         <pre><<shadow>> = <del>inset</del><ins><<color>></ins>? &amp;&amp; <<length>>{2,4} &amp;&amp; <del><<color>></del><ins>inset</ins>?</pre>
       </blockquote>
   <li>
-    Spread radius adjustment is only applied to outer shadows (and positive margins)--
-    i.e. cases where the radius of curvature grows, rather than shrinks.
+    Spread radius adjustment is only applied to shadows and margins
+    where the radius of curvature grows, rather than shrinks.
     (<a href="https://github.com/w3c/csswg-drafts/issues/1900">Issue 1900</a>)
       <blockquote>
         <p>To preserve the box's shape when spread is applied,
@@ -3067,8 +3071,10 @@ Changes since the 17 October 2017 Candidate Recommendation</h3>
         the <i>spread distance</i> (and flooring at zero).
         However, in order to create a sharper corner when the border radius is small
         <ins>(and thus ensure continuity between round and sharp corners)</ins>,
-        when the <i>border radius</i> is less than the <i>spread distance</i>,
-        the <i>spread distance</i> <ins>of an outer shadow</ins>
+        when the <i>border radius</i> is less than the <i>spread distance</i>
+        <ins>(or in the case of an inner shadow,
+        less than the absolute value of a negative <i>spread distance</i>)</ins>,
+        the <i>spread distance</i>
         is multiplied by the proportion 1 + (<var>r</var>-1)<sup>3</sup>,
         where <var>r</var> is the ratio of the border radius to the <i>spread distance</i>,
         in calculating the corner radii of the spread shadow shape.