Preventing FOUC

An element may not be initialised right away if your definitions are loaded after the document is parsed. In native custom elements, you can use the :defined pseudo-class to select all elements that have been upgraded, thus allowing you to do :not(:defined) to invert that. Since that only works in native, Skate adds a defined attribute so that you have a cross-browser way of dealing with FOUC and jank.

my-element {
  opacity: 1;
  transition: opacity .3s ease;
my-element:not([defined]) {
  opacity: 0;

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