[css-transforms-1] Use UA style instead of prose to handle the exception of transform-origin for SVG (w3c#920)

Author: smfr

css-transforms-1/Overview.bs

@@ -297,8 +297,6 @@ Media: visual
 Animatable: as <a href="https://drafts.csswg.org/css3-transitions/#animtype-simple-list">simple list</a> of <a href="https://drafts.csswg.org/css3-transitions/#animtype-lpcalc">length, percentage, or calc</a>
 </pre>
 
-The initial [=used value=] for SVG elements without associated CSS layout box is ''0 0''.
-
 The values of the 'transform' and 'transform-origin' properties are used to compute the [=transformation matrix=], as described above.
 
 If only one value is specified, the second value is assumed to be <a value for=transform-origin>center</a>. If one or two values are specified, the third value is assumed to be ''0px''.
@@ -322,6 +320,13 @@ If two or more values are defined and either no value is a keyword, or the only
 	::  Computes to ''50%'' (''left 50%'') for the horizontal position if the horizontal position is not otherwise specified, or ''50%'' (''top 50%'') for the vertical position if it is.
 </dl>
 
+For SVG elements without associated CSS layout box the initial [=used value=] is ''0 0'' as if the user agent style sheet contained:
+<pre>
+*:not(svg), *:not(foreignObject) &gt; svg {
+    transform-origin: 0 0;
+}
+</pre>
+
 The 'transform-origin' property is a <a>resolved value special case property like 'height'</a>. [[!CSSOM]]