Most references as below mention that Safari partially supports web components.
In detail, what web component features are not supported by Safari desktop and Safari iOS?
Is there a technical docs or specs about the issue?
Burn all old blogs!
Apple Safari supports Web Components, apart from Customized Built-In Elements.
After an ongoing debate since 2013, nearly everyone, who understand what OOP means, agrees Apple engineers are right on this one, not supporting Customized Built-In Elements
Customized Built-In Elements
extend from any existing HTML Element: <p>, <input>, etc..
customElements.define( "my-element", HTMLParagraphElement, { extends: "p" } );
If you want to understand the Apple Why,
read back in time to early December 2013 - Yes! Web Components are not a new fad!
Yes it would be cool to write <img is="queen-of-hearts"> (generating an IMG)
Autonomous Elements (extend from HTMLElement)
work 100% in all Modern Browsers
customElements.define( "playing-card", extends HTMLElement{ } );
We now have to do <playing-card is="queen-of-hearts">
<img> tag inside of it.<playing-card> with an <img>. . .
Deep dive into Web Components, React & Lit like BaseClasses:
Main differences between lit-element Web Components & React