[cssom-1] Minor grammar fix.

Author: emilio

cssom-1/Overview.bs

@@ -2083,7 +2083,7 @@ To <dfn export>serialize a CSS declaration block</dfn> <var>declaration block</v
      maps to one of the shorthand properties in <var>shorthands</var>.
      <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
+     <li>Append all <a>CSS declarations</a> in <var>longhands</var> that 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