Handle 'none' in DOMMatrix(transformList) constructor

Fixes #55.

Author: zcorpan

geometry/Overview.bs

@@ -692,15 +692,18 @@ the <dfn dfn-type=constructor dfn-for=DOMMatrix><code>DOMMatrix(<var>transformLi
 <ol>
   <li>If <var>transformList</var> is the empty string, set it to the string "<code lt>matrix(1, 0,
   0, 1, 0, 0)</code>".</li>
-  <li>Parse <var>transformList</var> by following the syntax description in “<a
-  href=https://drafts.csswg.org/css-transforms-1/#svg-syntax>Syntax of the SVG ‘transform’
-  attribute</a>” [[!CSS3-TRANSFORMS]] to a <<transform-list>>. If parsing is not successful, or any
-  <<transform-function>> has <<length>> values without <a spec='css-values'>absolute length</a>
-  units<!--For WD: <a spec='css-values-3'>absolute length units</a>-->, or any keyword other than
-  ''transform/none'' is used, throw a {{SyntaxError}} exception.</li>
-  <li>Let <var>2dTransform</var> track the 2D/3D dimension status of the <<transform-list>>.
+  <li>Parse <var>transformList</var> into <var>parsedValue</var> by following the syntax description in
+  “<a href=https://drafts.csswg.org/css-transforms-1/#svg-syntax>Syntax of the SVG ‘transform’
+  attribute</a>” [[!CSS3-TRANSFORMS]] to a <<transform-list>> or the keyword ''transform/none''. If
+  parsing is not successful, or any <<transform-function>> has <<length>> values without <a
+  spec='css-values'>absolute length</a> units<!--For WD: <a spec='css-values-3'>absolute length
+  units</a>-->, or any keyword other than ''transform/none'' is used, throw a {{SyntaxError}}
+  exception.</li>
+  <li>If <var>parsedValue</var> is ''transform/none'', set <var>parsedValue</var> to a
+  <<transform-list>> containing a single identity matrix</li>
+  <li>Let <var>2dTransform</var> track the 2D/3D dimension status of <var>parsedValue</var>.
     <dl class=switch>
-      <dt>If the <<transform-list>> consists of any <a href='https://drafts.csswg.org/css-transforms-1/#three-d-transform-functions'>3D Transform functions</a></dt>
+      <dt>If <var>parsedValue</var> consists of any <a href='https://drafts.csswg.org/css-transforms-1/#three-d-transform-functions'>3D Transform functions</a></dt>
       <dd>Set <var>2dTransform</var> to <code>false</code>.</dd>
       <dt>Otherwise</dt>
       <dd>Set <var>2dTransform</var> to <code>true</code>.</dd>