[cssom] Support webkitFoo IDL attributes (in addition to WebkitFoo)

The camel-cased attributes rule already supports `WebkitFoo`, but
WebKit and Blink also support a variant `webkitFoo` with lowercase `w`.

Ref. whatwg/compat#21
Ref. whatwg/compat#20

Author: zcorpan

cssom/Overview.bs

@@ -16,7 +16,8 @@ Former Editor: Glenn Adams, Cox Communications, Inc. http://www.cox.com, gle
 Former Editor: Anne van Kesteren, Opera Software ASA http://www.opera.com, annevk@annevk.nl, https://annevankesteren.nl/
 !Issues list: <a href="https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&amp;component=CSSOM&amp;resolution=---">Bugzilla</a> (<a href="https://www.w3.org/Bugs/Public/enter_bug.cgi?product=CSS&amp;component=CSSOM">file a bug</a>)
 Abstract: CSSOM defines APIs (including generic parsing and serialization rules) for Media Queries, Selectors, and of course CSS itself.
-Ignored Terms: EmptyString, mediaText, cssText, InvalidCharacterError, SecurityError, SyntaxError, IndexSizeError, HierarchyRequestError, InvalidStateError, InvalidModificationError, NoModificationAllowedError, CORS-same-origin, group of selectors, list of css page selectors, CSSCharsetRule, ProcessingInstruction, EventTarget, EventListener, Event, EventInit, Element, Range, Node, Text, style, CSSFontFaceRule
+Ignored Terms: EmptyString, mediaText, cssText, InvalidCharacterError, SecurityError, SyntaxError, IndexSizeError, HierarchyRequestError, InvalidStateError, InvalidModificationError, NoModificationAllowedError, CORS-same-origin, group of selectors, list of css page selectors, CSSCharsetRule, ProcessingInstruction, EventTarget, EventListener, Event, EventInit, Element, Range, Node, Text, style, CSSFontFaceRule, -webkit-transform
+Ignored Vars: m1, m2, camel_cased_attribute, webkit_cased_attribute, dashed_attribute
 </pre>
 
 <pre class='anchors'>
@@ -104,6 +105,7 @@ spec:css-color-4; type:property; text:color
 spec:css-position-3; type:property; text:left
 spec:html5; type:element; text:style
 spec:css-namespaces-3; type:dfn; text:namespace prefix
+spec:dom-ls; type:interface; text:Document
 </pre>
 
 <script src=https://resources.whatwg.org/file-bug.js async></script>
@@ -2408,6 +2410,30 @@ exceptions thrown must be re-thrown.
 
 <div class=example>For example, for the 'font-size' property there would be a <code>fontSize</code> IDL attribute.</div>
 
+For each CSS property <var>property</var> that is a <a>supported CSS property</a> and that begins
+with the string <code>-webkit-</code>, the following partial interface applies where
+<var>webkit-cased attribute</var> is obtained by running the <a>CSS property to IDL attribute</a>
+algorithm for <var>property</var>, with the <i>lowercase first</i> flag set.
+
+<pre class="idl extract">partial interface CSSStyleDeclaration {
+  [TreatNullAs=EmptyString] attribute DOMString _<var>webkit_cased_attribute</var>;
+};</pre>
+
+The <dfn attribute for=CSSStyleDeclaration><var>webkit-cased attribute</var></dfn> attribute, on
+getting, must return the result of invoking {{CSSStyleDeclaration/getPropertyValue()}} with the
+argument being the result of running the <a>IDL attribute to CSS property</a> algorithm for
+<var>webkit-cased attribute</var>, with the <i>dash prefix</i> flag set.
+
+Setting the <code attribute for=CSSStyleDeclaration><var>webkit-cased attribute</var></code>
+attribute must invoke {{CSSStyleDeclaration/setProperty()}} with the first argument being the result
+of running the <a>IDL attribute to CSS property</a> algorithm for <var>webkit-cased attribute</var>,
+with the <i>dash prefix</i> flag set, as second argument the given value, and no third argument.
+Any exceptions thrown must be re-thrown.
+
+<div class=example>For example, if the user agent supports the '-webkit-transform' property, there
+would be a <code>webkitTransform</code> IDL attribute. There would also be a
+<code>WebkitTransform</code> IDL attribute because of the rules for camel-cased attributes.</div>
+
 For each CSS property <var>property</var> that is a <a>supported CSS property</a>,
 except for properties that have no "<code>-</code>" (U+002D) in the property name,
 the following partial interface applies where <var>dashed attribute</var> is <var>property</var>.
@@ -2431,14 +2457,16 @@ exceptions thrown must be re-thrown.
  <pre><var>element</var>.style['font-size'];</pre>
 </div>
 
-The <dfn>CSS property to IDL attribute</dfn> algorithm for <var>property</var> is as
-follows:
+The <dfn>CSS property to IDL attribute</dfn> algorithm for <var>property</var>, optionally with a
+<i>lowercase first</i> flag set, is as follows:
 
 <ol>
  <li>Let <var>output</var> be the empty string.
 
  <li>Let <var>uppercase next</var> be unset.
 
+ <li>If the <i>lowercase first</i> flag is set, remove the first character from <var>property</var>.
+
  <li>For each character <var>c</var> in <var>property</var>:
 
   <ol>
@@ -2453,12 +2481,14 @@ follows:
  <li>Return <var>output</var>.
 </ol>
 
-The <dfn>IDL attribute to CSS property</dfn> algorithm for <var>attribute</var> is as
-follows:
+The <dfn>IDL attribute to CSS property</dfn> algorithm for <var>attribute</var>, optionally with a
+<i>dash prefix</i> flag set, is as follows:
 
 <ol>
  <li>Let <var>output</var> be the empty string.
 
+ <li>If the <i>dash first</i> flag is set, append "<code>-</code>" (U+002D) to <var>output</var>.
+
  <li>For each character <var>c</var> in <var>attribute</var>:
 
   <ol>