These selectors are evaluated globally, checked against to EVERY NODE in the DOM. You might as well write

  *[am-Button]

just to hammer home to the reader that performance is going to be a problem with this selector approach on any complex DOM.

If I recall, CSS performance can roughly be gauged by the key selector (right most thingy on the selector) like this:

  * inline styles = fastest (ok, this doesn't have a selector, but it evaluates fast!)
  * id = fastest-ish
  * class = fast
  * tagname = slower
  * any/everything else evaluated like global = super slowest