Try to address <http://lists.w3.org/Archives/Public/www-style/2011Sep/0172.html> by sizing the image first to the border image area.

Author: fantasai

css3-background/Overview.html

@@ -3079,7 +3079,10 @@ <h3 id=the-border-image-slice><span class=secno>6.2. </span>The &lsquo;<a
    <dt><var>&lt;number&gt;</var>
 
    <dd>Numbers represent pixels in the image (if the image is a raster image)
-    or vector coordinates (if the image is a vector image).
+    or vector coordinates (if the image is a vector image). If the image must
+    be sized to determine the slices (for example, for SVG images with no
+    intrinsic size), then it is sized to the <a
+    href="#border-image-area"><i>border image area</i></a>.
 
    <dt><dfn id=border-image-slice-fill>&lsquo;<code
     class=css>fill</code>&rsquo;</dfn>

css3-background/Overview.src.html

@@ -2246,7 +2246,10 @@ <h3 id="the-border-image-slice">The 'border-image-slice' property</h3>
 
   <dt><var>&lt;number&gt;</var>
   <dd>Numbers represent pixels in the image (if the image is a raster
-    image) or vector coordinates (if the image is a vector image).
+    image) or vector coordinates (if the image is a vector image). If
+    the image must be sized to determine the slices (for example, for
+    SVG images with no intrinsic size), then it is sized to the
+    <i>border image area</i>.
 
   <dt><dfn id="border-image-slice-fill">''fill''</dfn>
   <dd>The ''fill'' keyword, if present, causes the middle part of