What web component features are not supported by Safari desktop and Safari iOS?

Burn all old blogs!

Apple Safari supports Web Components, apart from Customized Built-In Elements.
After an ongoing debate since 2013, nearly everyone agrees Apple engineers are right on this one.

Not yet supported by Apple (Google proposals, implemented in Chromium):

• Constructable StyleSheets : https://web.dev/constructable-stylesheets/

• declarative shadowDOM : https://web.dev/declarative-shadow-dom/

(almost) implemented by Apple:

• shadowParts (not available yet in Safari 15.4 march 2022, will be available in next release)

Will never be implemented by Apple:

• Customized Built-In Elements
  extend from any existing HTML Element: <p>, <input>, etc..

  If you want to understand the Apple Why,
  read back in time to 2013 - Yes! Web Components are not a new fad!

Autonomous Elements (extend from HTMLElement)
work 100% in all Modern Browsers

At this time of writing, see: https://custom-elements-everywhere.com/

• Angular is 100% compatible

• Svelte is 100% compatible

• Vue is 91% compatible (Vue doesn't listen to Capitalized Eventnames)

• React is 29% incompatible

  StackOverflow Deep dive into Web Components, React & Lit like BaseClasses:
  Main differences between lit-element & React

Javascript regexp variable length lookbehind is not supported by Safari but works in Chrome, Edge and Firefox. Maybe international agreements/standards say Safari doesn't need to support it (yet) but it's a pain nonetheless.