[cssom] Don't serialize a shorthand if there are longhands with other property group / mapping logic in between the longhands of that shorthand.

Per #3030.

Author: emilio

cssom-1/Overview.bs

@@ -2088,6 +2088,9 @@ To <dfn export>serialize a CSS declaration block</dfn> <var>declaration block</v
      <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>If there's any declaration in <var>declaration block</var> in between the first and the last longhand in <var>current longhands</var>
+     which belongs to the same [=logical property group=], but has a different [=mapping logic=] as any of the longhands in <var>current longhands</var>,
+     and is not in <var>current longhands</var>, 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