[cssom] Reduce by one the level of indentation of the "serialize a CSS declaration block" algo.

By moving `longhands` into the loop we can also avoid removing from it
dynamically when serializing a given shorthand.

Author: emilio

cssom-1/Overview.bs

@@ -2074,30 +2074,27 @@ To <dfn export>serialize a CSS declaration block</dfn> <var>declaration block</v
    <li>Let <var>property</var> be <var>declaration</var>'s <a for="CSS declaration">property name</a>.
    <li>If <var>property</var> is in <var>already serialized</var>, continue with the steps labeled <i>declaration loop</i>.
    <li>If <var>property</var> maps to one or more shorthand properties, let <var>shorthands</var> be an array of those shorthand properties, in
-   <a>preferred order</a>, and follow these substeps:
+   <a>preferred order</a>.
+   <li><i>Shorthand loop</i>: For each <var>shorthand</var> in <var>shorthands</var>, follow these substeps:
     <ol>
      <li>Let <var>longhands</var> be an array consisting of all <a>CSS declarations</a> in <var>declaration block</var>'s
      <a for="CSSStyleDeclaration">declarations</a> that that are not in <var>already serialized</var> and have a
      <a for="CSS declaration">property name</a> that
      maps to one of the shorthand properties in <var>shorthands</var>.
-     <li><i>Shorthand loop</i>: For each <var>shorthand</var> in <var>shorthands</var>, follow these substeps:
-      <ol>
-       <li>If all properties that map to <var>shorthand</var> are not present in <var>longhands</var>, continue with the steps labeled <i>shorthand loop</i>.
-       <li>Let <var>current longhands</var> be an empty array.
-       <li>Append all <a>CSS declarations</a> in <var>longhands</var> have a
-       <a for="CSS declaration">property name</a> that maps to <var>shorthand</var> to <var>current longhands</var>.
-       <li>If there is one or more <a>CSS declarations</a> in <var>current longhands</var> have their
-       <a for="CSS declaration">important flag</a> set and one or more with it unset, continue with
-       the steps labeled <i>shorthand loop</i>.
-       <li>Let <var>value</var> be the result of invoking <a>serialize a CSS value</a> of <var>current longhands</var>.
-       <li>If <var>value</var> is the empty string, continue with the steps labeled <i>shorthand loop</i>.
-       <li>Let <var>serialized declaration</var> be the result of invoking <a>serialize a CSS declaration</a> with property name
-       <var>shorthand</var>, value <var>value</var>, and the <i>important</i> flag set if the <a>CSS declarations</a> in
-       <var>current longhands</var> have their <a for="CSS declaration">important flag</a> set.
-       <li>Append <var>serialized declaration</var> to <var>list</var>.
-       <li>Append the property names of all items of <var>current longhands</var> to <var>already serialized</var>.
-       <li>Remove the items present in <var>current longhands</var> from <var>longhands</var>.
-      </ol>
+     <li>If all properties that map to <var>shorthand</var> are not present in <var>longhands</var>, continue with the steps labeled <i>shorthand loop</i>.
+     <li>Let <var>current longhands</var> be an empty array.
+     <li>Append all <a>CSS declarations</a> in <var>longhands</var> have a
+     <a for="CSS declaration">property name</a> that maps to <var>shorthand</var> to <var>current longhands</var>.
+     <li>If there is one or more <a>CSS declarations</a> in <var>current longhands</var> have their
+     <a for="CSS declaration">important flag</a> set and one or more with it unset, continue with
+     the steps labeled <i>shorthand loop</i>.
+     <li>Let <var>value</var> be the result of invoking <a>serialize a CSS value</a> of <var>current longhands</var>.
+     <li>If <var>value</var> is the empty string, continue with the steps labeled <i>shorthand loop</i>.
+     <li>Let <var>serialized declaration</var> be the result of invoking <a>serialize a CSS declaration</a> with property name
+     <var>shorthand</var>, value <var>value</var>, and the <i>important</i> flag set if the <a>CSS declarations</a> in
+     <var>current longhands</var> have their <a for="CSS declaration">important flag</a> set.
+     <li>Append <var>serialized declaration</var> to <var>list</var>.
+     <li>Append the property names of all items of <var>current longhands</var> to <var>already serialized</var>.
     </ol>
    <li>If <var>property</var> is in <var>already serialized</var>, continue with the steps labeled <i>declaration loop</i>.
    <li>Let <var>value</var> be the result of invoking <a>serialize a CSS value</a> of <var>declaration</var>.