Otherwise we can have a situation like this:

  <style>
    .notmatching > #foo {}
  </style>
  <span id="foo"></span>
  <span></span>

and the style sharing cache lookup for the second <span> would try to revalidate
against the cached value for the first <span>, but end up failing asserts about
the two elements matching lists of revalidation selectors that have the same
length.