Remove nonsensical references to 'position' from offset-anchor. Generally simplify. Add issue for which box to use.

Author: tabatkins

motion-1/Overview.bs

@@ -949,32 +949,38 @@ Define an anchor point: The 'offset-anchor' property {#offset-anchor-property}
 
 <pre class=propdef>
 Name: offset-anchor
-Applies to: <a href="https://drafts.csswg.org/css-transforms-1/#transformable-element">transformable elements</a>
+Applies to: [=transformable elements=]
 Value: auto | <<position>>
 Initial: auto
 Media: visual
 Inherited: no
-Percentages: Relative to the width and the height of a box
-Computed value: For <<length>> the absolute value, otherwise a percentage.
+Percentages: relative to the width and the height of the element's [=reference box=]
+Computed value: the ''offset-anchor/auto'' keyword or a computed <<position>>
 Animation type: by computed value
 </pre>
 
-Defines an <a>anchor point</a> of the box positioned along the <a>offset path</a>. The <dfn>anchor point</dfn> specifies the point of the box which is to be considered as the point that is moved along the <a>offset path</a>.
+Defines the box's <dfn export>anchor point</dfn>--
+the point that is positioned along the [=offset path=].
 
 Values have the following meanings:
 <dl dfn-for="offset-anchor" dfn-type="value">
-    : <var>auto</var>
-    ::
-        Computes to the value from 'offset-position', provided 'offset-path' is <a href="#offsetpath-none">none</a> and 'offset-position' is not ''auto''. Otherwise, computes to the value from 'transform-origin'. When ''auto'' is given to 'offset-anchor', and 'offset-path' is <a href="#offsetpath-none">none</a>, 'offset-position' behaves similar to 'background-position'.
-    : <<position>>
+    : <dfn>auto</dfn>
+    :: The [=anchor point=] is the same as
+        the point indicated by 'transform-origin'.
+
+        Specifically, the [=computed value=] of 'transform-origin'
+        is resolved as a <<position>>
+        against the element's [=reference box=].
+
+    : <dfn><<position>></dfn>
     ::
-        : <<percentage>>
-        ::
-            A percentage for the horizontal offset is relative to width of content area. A percentage for the vertical offset is relative to the height of the content area. For example, with a value pair of '100%, 0%', an <a>anchor point</a> is on the upper right corner of the box.
-        : <<length>>
-        :: A length value gives a length offset from the upper left corner of the box's content area.
+        The [=anchor point=] is the result of resolving the <<position>>
+        against the element's [=reference box=].
 </dl>
 
+Issue: Which box this is resolved against is being discussed in
+<a href="https://github.com/w3c/fxtf-drafts/issues/503">Issue 503</a>.
+
 <div class=example>
 The following explains how to set the <a>anchor point</a> of the box.