For a long time I didn't know what to do with web components...No shadow DOM, no templates, just the regular old DOM, which we now get to call the much cooler sounding 'light DOM'.
Custom elements have the very cool ability manage themselves via connectedCallback and disconnectedCallback. These are the unsung heroes of the spec; the things that make web components the can-do workhorses that they are.
Progressive enhancement is straightforward. Any markup you add inside your custom element is just... there, javascript or no. Wrap standard interactive elements like details, textarea, input, etc in custom elements that enhances them, and you've got yourself a solid progressive enhancement story.
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