Selectors should be picked up regardless of whether a declaration block
is present.

The "it does not confuse custom elements with properties" test can be
delegated to another suite so we don't need this dedicated spec.

https://www.w3.org/TR/custom-elements/#valid-custom-element-name

* They start with a lowercase ASCII letter, ensuring that the HTML
  parser will treat them as tags instead of as text.
* They do not contain any uppercase ASCII letters, ensuring that the
  user agent can always treat HTML elements ASCII-case-insensitively.
* They contain a hyphen, used for namespacing and to ensure forward
  compatibility (since no elements will be added to HTML, SVG, or
  MathML with hyphen-containing local names in the future).