[cssom] Use [TreatNullAs=EmptyString] on CSSStyleDeclaration's properties. Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=26817

Author: Simon Pieters

cssom/Overview.html

@@ -2353,7 +2353,7 @@ <h4 id="the-cssstyledeclaration-interface"><span class="secno">6.6.1 </span>The
   void <a href="#dom-cssstyledeclaration-setpropertypriority" title="dom-CSSStyleDeclaration-setPropertyPriority">setPropertyPriority</a>(DOMString property, [TreatNullAs=EmptyString] DOMString priority);
   DOMString <a href="#dom-cssstyledeclaration-removeproperty" title="dom-CSSStyleDeclaration-removeProperty">removeProperty</a>(DOMString property);
   readonly attribute <a href="#cssrule">CSSRule</a>? <a href="#dom-cssstyledeclaration-parentrule" title="dom-CSSStyleDeclaration-parentRule">parentRule</a>;
-           attribute DOMString <a href="#dom-cssstyledeclaration-cssfloat" title="dom-CSSStyleDeclaration-cssFloat">cssFloat</a>;<!-- setProperty's value arg has [TreatNullAs=EmptyString] -->
+  [TreatNullAs=EmptyString] attribute DOMString <a href="#dom-cssstyledeclaration-cssfloat" title="dom-CSSStyleDeclaration-cssFloat">cssFloat</a>;
 };</pre>
 
 <p>The object's <a class="external" data-anolis-spec="webidl" href="http://heycam.github.io/webidl/#dfn-supported-property-indices">supported property indices</a> are the numbers in the range zero to one less than the number of
@@ -2559,7 +2559,7 @@ <h4 id="the-cssstyledeclaration-interface"><span class="secno">6.6.1 </span>The
 <var>property</var>.</p>
 
 <pre class="idl extract">partial interface <a href="#cssstyledeclaration">CSSStyleDeclaration</a> {
-           attribute DOMString _<a href="#dom-cssstyledeclaration-camel-cased-attribute" title="dom-CSSStyleDeclaration-camel-cased-attribute"><var>camel-cased attribute</var></a>;
+  [TreatNullAs=EmptyString] attribute DOMString _<a href="#dom-cssstyledeclaration-camel-cased-attribute" title="dom-CSSStyleDeclaration-camel-cased-attribute"><var>camel-cased attribute</var></a>;
 };</pre>
 
 <p>The <dfn id="dom-cssstyledeclaration-camel-cased-attribute" title="dom-CSSStyleDeclaration-camel-cased-attribute"><code><var>camel-cased attribute</var></code></dfn> attribute, on getting, must return the
@@ -2580,7 +2580,7 @@ <h4 id="the-cssstyledeclaration-interface"><span class="secno">6.6.1 </span>The
 the following partial interface applies where <var>dashed attribute</var> is <var>property</var>.</p>
 
 <pre class="idl extract">partial interface <a href="#cssstyledeclaration">CSSStyleDeclaration</a> {
-           attribute DOMString _<a href="#dom-cssstyledeclaration-dashed-attribute" title="dom-CSSStyleDeclaration-dashed-attribute"><var>dashed attribute</var></a>;
+  [TreatNullAs=EmptyString] attribute DOMString _<a href="#dom-cssstyledeclaration-dashed-attribute" title="dom-CSSStyleDeclaration-dashed-attribute"><var>dashed attribute</var></a>;
 };</pre>
 
 <p>The <dfn id="dom-cssstyledeclaration-dashed-attribute" title="dom-CSSStyleDeclaration-dashed-attribute"><code><var>dashed attribute</var></code></dfn> attribute, on getting, must return the

cssom/Overview.src.html

@@ -2270,7 +2270,7 @@ <h4>The <code title>CSSStyleDeclaration</code> Interface</h4>
   void <span title=dom-CSSStyleDeclaration-setPropertyPriority>setPropertyPriority</span>(DOMString property, [TreatNullAs=EmptyString] DOMString priority);
   DOMString <span title=dom-CSSStyleDeclaration-removeProperty>removeProperty</span>(DOMString property);
   readonly attribute <span>CSSRule</span>? <span title=dom-CSSStyleDeclaration-parentRule>parentRule</span>;
-           attribute DOMString <span title=dom-CSSStyleDeclaration-cssFloat>cssFloat</span>;<!-- setProperty's value arg has [TreatNullAs=EmptyString] -->
+  [TreatNullAs=EmptyString] attribute DOMString <span title=dom-CSSStyleDeclaration-cssFloat>cssFloat</span>;
 };</pre>
 
 <p>The object's <span data-anolis-spec=webidl>supported property indices</span> are the numbers in the range zero to one less than the number of
@@ -2476,7 +2476,7 @@ <h4>The <code title>CSSStyleDeclaration</code> Interface</h4>
 <var>property</var>.</p>
 
 <pre class="idl extract">partial interface <span>CSSStyleDeclaration</span> {
-           attribute DOMString _<span title=dom-CSSStyleDeclaration-camel-cased-attribute><var>camel-cased attribute</var></span>;
+  [TreatNullAs=EmptyString] attribute DOMString _<span title=dom-CSSStyleDeclaration-camel-cased-attribute><var>camel-cased attribute</var></span>;
 };</pre>
 
 <p>The <dfn title=dom-CSSStyleDeclaration-camel-cased-attribute><code><var>camel-cased attribute</var></code></dfn> attribute, on getting, must return the
@@ -2497,7 +2497,7 @@ <h4>The <code title>CSSStyleDeclaration</code> Interface</h4>
 the following partial interface applies where <var>dashed attribute</var> is <var>property</var>.</p>
 
 <pre class="idl extract">partial interface <span>CSSStyleDeclaration</span> {
-           attribute DOMString _<span title=dom-CSSStyleDeclaration-dashed-attribute><var>dashed attribute</var></span>;
+  [TreatNullAs=EmptyString] attribute DOMString _<span title=dom-CSSStyleDeclaration-dashed-attribute><var>dashed attribute</var></span>;
 };</pre>
 
 <p>The <dfn title=dom-CSSStyleDeclaration-dashed-attribute><code><var>dashed attribute</var></code></dfn> attribute, on getting, must return the