s/not drawn/invisible/ for less awkward wording

Author: fantasai

css3-background/Overview.html

@@ -3163,7 +3163,7 @@ <h3 id=the-border-image-source><span class=secno>6.1. </span>Image Source:
    additional background layer for the element. If the value is &lsquo;<code
    class=css>none</code>&rsquo; or if the image cannot be displayed (or the
    property doesn't apply), the border styles will be used; otherwise the
-   element's borders are not drawn and the border image is drawn as described
+   element's borders are invisible and the border image is drawn as described
    in the sections below.
 
   <h3 id=the-border-image-slice><span class=secno>6.2. </span>Image Slicing:

css3-background/Overview.src.html

@@ -2279,7 +2279,7 @@ <h3 id="the-border-image-source">Image Source: the 'border-image-source' propert
 'border-style' properties and as an additional background layer for the
 element. If the value is ''none'' or if the image cannot be displayed
 (or the property doesn't apply), the border styles will be used;
-otherwise the element's borders are not drawn and the border image is
+otherwise the element's borders are invisible and the border image is
 drawn as described in the sections below.
 
 <h3 id="the-border-image-slice">Image Slicing: the 'border-image-slice' property</h3>