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“If you just sit and observe, you will see how restless your mind is. If you try to calm it, it only makes it worse, but over time it does calm, and when it does, there’s room to hear more subtle things. You see so much more than you could see before. It’s a discipline; you have to practice it.”
Steve Jobs

:defined

The :defined CSS pseudo-class represents any element that has been defined. This includes any standard element built in to the browser, and custom elements that have been successfully defined (i.e. with the CustomElementRegistry.define() method).
:defined css reference