[cssom] Specify CSSStyleDeclaration#getPropertyValue()

Author: Simon Pieters

cssom/idl/CSSStyleDeclaration.idl

@@ -27,7 +27,7 @@ interface CSSStyleDeclaration {
   attribute DOMString cssText;
 
   [Documentation=
-   "<p>The {@name} {@type} must return the number of declarations in the <span>collection of CSS declarations</span>.</p>"
+   "<p>The {@name} {@type} must return the number of declarations in the <span>CSS declaration block declarations</span>.</p>"
   ]
   readonly attribute unsigned long length;
 
@@ -37,7 +37,15 @@ interface CSSStyleDeclaration {
   DOMString item(unsigned long index);
 
   [Documentation=
-   "<p>The {@name} {@type} must ...</p>"
+   "<p>The {@name} {@type} must run these steps:</p>\
+    <ol>\
+     <li><p>If <var>property</var> is an <span data-anolis-spec=dom>ASCII\
+     case-insensitive</span> match for a property of a declaration in the\
+     <span>CSS declaration block declarations</span>, return the\
+     <span title='serialize a CSS value'>serialization</span> of that declaration's\
+     value.\
+     <li><p>Return the empty string.\
+    </ol>"
   ]
   DOMString getPropertyValue(DOMString property);
 
@@ -76,7 +84,7 @@ interface CSSStyleDeclaration {
      <li><p>If the <span>CSS declaration block readonly flag</span> is set, <span data-anolis-spec=dom title=concept-throw>throw</span> a\
        '<code data-anolis-spec=dom>NoModificationAllowedError</code>' and terminate these steps.</li>\
      <li><p>If <var>property</var> is an <span data-anolis-spec=dom>ASCII case-insensitive</span> match for a property of a declaration in the\
-       <span>collection of CSS declarations</span> remove the declaration.</p></li>\
+       <span>CSS declaration block declarations</span> remove the declaration.</p></li>\
     </ol>"
   ]
   DOMString removeProperty(DOMString property);