Tweak parsing of transformList

For DOMMatrix(transformList) and setMatrixValue(transformList),
the empty string is the identity matrix, and keywords other than
'none' throw.

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=29446.

Author: zcorpan

geometry/Overview.bs

@@ -666,7 +666,14 @@ The <dfn dfn-type=constructor dfn-for=DOMMatrixReadOnly><code>DOMMatrixReadOnly(
 the <dfn dfn-type=constructor dfn-for=DOMMatrix><code>DOMMatrix(<var>transformList</var>)</code></dfn> constructors, when invoked with a {{DOMString}} argument, must follow these steps:
 
 <ol>
-  <li>Parse <var>transformList</var> by following the syntax description in “<a href=https://www.w3.org/TR/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>-->, throw a {{SyntaxError}} exception.</li>
+  <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://www.w3.org/TR/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>>.
     <dl class=switch>
       <dt>If the <<transform-list>> consists of any <a href='https://www.w3.org/TR/css-transforms-1/#three-d-transform-functions'>3D Transform functions</a></dt>
@@ -1127,7 +1134,14 @@ Note: Authors who use chained method calls are recommended to use mutable transf
   <dt><dfn>setMatrixValue(<var>transformList</var>)</dfn></dt>
   <dd>
     <ol>
-      <li>Parse <var>transformList</var> by following the syntax description in “<a href=https://www.w3.org/TR/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>-->, throw a {{SyntaxError}} exception.</li>
+      <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://www.w3.org/TR/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>Set <a>is2D</a> to <code>false</code> if the <<transform-list>> consists of any <a href='https://www.w3.org/TR/css-transforms-1/#three-d-transform-functions'>3D Transform functions</a>. Otherwise set <a>is2D</a> to <code>true</code>.</li>
       <li>Transform all <<transform-function>>s to 4x4 matrices by following the “<a href=https://www.w3.org/TR/css-transforms-1/#mathematical-description>Mathematical Description of Transform Functions</a>” [[!CSS3-TRANSFORMS]].</li>
       <li>Post-multiply all matrices from left to right to a combined 4x4 matrix.</li>