Add reference box to motion-path syntax

Author: dirkschulze

motion-1/index.bs

@@ -51,7 +51,7 @@ ISSUE: Add more details and examples.
 
 <pre class='propdef'>
 Name: motion-path
-Value: <<basic-shape>> | <<path()>> | <<url>> | none
+Value: <<url>> | [ <<basic-shape>> | <<path()>> ] || <<geometry-box>> | none
 Initial: none
 Applies to: All elements. In SVG, it applies to <a href="">container elements</a> excluding the <a href="">defs</a> element and all <a href="">graphics elements</a>
 Inherited: no
@@ -100,7 +100,9 @@ A reference that fails to download, is not a reference to an SVG <a href="">grap
 
 See the section <a href="#motion-processing">“Motion processing”</a> for how to process a motion path.
 
-ISSUE: Which coordinate system do we use for elements with associated layout box? The canvas boundaries? The viewport boundaries? An element reference box? Do we need to allow keywords to specify this box?
+For SVG elements without associated CSS layout box, the <i>used value</i> for <a value for=mask-clip>content-box</a>, <a value for=mask-clip>padding-box</a>, <a value for=mask-clip>border-box</a> and <a value for=mask-clip>margin-box</a> is <a value for=clip-path>fill-box</a>.
+
+For elements with associated CSS layout box, the <i>used value</i> for <a value for=clip-path>fill-box</a>, <a value for=clip-path>stroke-box</a> and <a value for=clip-path>view-box</a> is <a value for=mask-clip>border-box</a>.
 
 <h3 id="motion-position">Position on motion path: The 'motion-position' property</h3>