The WebKit team's post on Interop 2026 lists the twenty areas the browser makers have agreed to make consistent this year, and two of them matter to anyone who maintains a component library.
The first is scoped custom element registries. Today the registry for custom elements is global: one definition per tag name across the entire page. That is a problem for two versions of a library on one page, a microfrontend with its own elements, or a third-party widget using the same tag. Scoped registries let different parts of an app, or different shadow roots, define the same tag independently. The second is scroll-driven animations, which move scroll effects from JavaScript into CSS.
I think scoped registries are the one to watch, because they change what a component library can promise. The global registry has always meant that a library could be broken by whatever else was on the page. A scoped registry makes a library's elements its own business.
My view is that a team maintaining a library across many applications should be tracking this one, with a note in the backlog for the day it is reliably available, because it removes a bug with no clean fix today.
What is the worst collision your team has hit between components that were never meant to share a page?
Photo source: https://photos.robertstowe.com/wieliczka-salt-mine