Flash of Undefined Custom Elements
Fleeting- External reference: https://www.abeautifulsite.net/posts/flash-of-undefined-custom-elements/
A Web Component Story
custom elements should be when the page loads. This is not dissimilar to FOUC, which occurs when HTML is displayed before the stylesheet has loaded.
— https://www.abeautifulsite.net/posts/flash-of-undefined-custom-elements/