Declarative shadow DOM

[o-t-w]

I realise the website is called custom elements everywhere and not shadow DOM everywhere, but from my own small amount of testing, declarative shadow DOM does not play nicely with JS frameworks and it would be helpful to document that.

[o-t-w]

Example issues:

• Bug: Reconciler cannot handle Declarative Shadow DOM (DSD) facebook/react#26071
• Declarative shadow DOM does not work  vercel/next.js#53356