FAST Element v3 builds on native web platform features. The baseline client-rendered runtime expects a browser Window environment with autonomous custom elements, Shadow DOM, requestAnimationFrame, and native globalThis support. Worker contexts, server-side JavaScript runtimes, and other non-window hosts are outside the supported runtime boundary for FAST Element's client runtime. Optional capabilities, such as Declarative Shadow DOM hydration and constructable stylesheets, use newer browser features when they are available.

Baseline runtime support

The following browser versions have native support for the baseline features used by client-rendered @microsoft/fast-element components:

  • Microsoft Edge 79+
  • Mozilla Firefox 65+
  • Google Chrome 71+
  • Apple Safari 12.1+
  • Opera 58+
  • iOS Safari 12.2+
  • Android Browser 81+
  • Opera Mobile 54+
  • Chrome for Android 81+
  • Firefox for Android 68+
  • Samsung Internet 10.0+

FAST Element v3 does not include platform polyfills. If an application must run below these minimums, load standards-compatible polyfills before importing FAST. The optional features in the matrix below do not change this baseline list unless an application opts into those features.

Feature support matrix

Feature Browser support guidance Fallback and application requirements
Web Components / custom elements Supported by the baseline browsers above for autonomous custom elements. FAST Element does not use customized built-in elements. No FAST fallback. Browsers must provide native custom elements, or the application must load compatible platform polyfills before defining FAST elements.
Shadow DOM Supported by the baseline browsers above. FAST creates an open shadow root by default for templates and style encapsulation. No FAST Shadow DOM polyfill. Components that intentionally set shadowOptions: null can render to light DOM, but default FAST Element authoring expects browser Shadow DOM support.
requestAnimationFrame Required by the FAST Element client runtime for asynchronous DOM update batching through Updates. Supported by the baseline browser Window environments above. FAST Element does not support worker or non-window runtimes where requestAnimationFrame is unavailable. Run FAST Element in the browser window and use server-rendering tools only to produce HTML for the browser to consume.
Declarative Shadow DOM Optional for client-only rendering. Hydration requires browser support for <template shadowrootmode>, such as Chrome/Edge 111+, Firefox 123+, Safari/iOS Safari 16.4+, Opera 97+, Samsung Internet 22+, and current Chromium-based mobile browsers. FAST hydrates an existing shadow root; it does not make unsupported browsers parse Declarative Shadow DOM. Older browsers need an application-level DSD transform/polyfill before components connect, or they should use client rendering.
adoptedStyleSheets / constructable stylesheets Supported in Chrome 73+, Edge 79+, Firefox 101+, Safari/iOS Safari 16.4+, Opera 60+, Samsung Internet 11.1+, and current Chromium-based mobile browsers. FAST falls back to injected <style> elements for string-based css styles when adopted stylesheets are unavailable. Passing CSSStyleSheet instances directly requires browser constructable stylesheet support.
Trusted Types Not part of the FAST Element baseline. Available in Chrome/Edge 83+, Firefox 148+, Safari/iOS Safari 26+, Opera 69+, Samsung Internet 13+, and matching current Chromium-based mobile browsers. FAST creates a Trusted Types policy when globalThis.trustedTypes exists and otherwise uses its string-based DOM policy. Apps that enforce require-trusted-types-for must configure an appropriate DOM policy before templates compile.
globalThis Required by FAST Element v3. The baseline browser versions above include native globalThis support. FAST Element v3 no longer polyfills globalThis. Load a globalThis polyfill before importing FAST in older browsers.

SSR and hydration notes

Client-rendered FAST Element components only need the baseline runtime features. Server-side rendering and hydration add browser and application requirements:

  • The server must emit FAST-compatible hydration markers and Declarative Shadow DOM templates.
  • The browser must parse Declarative Shadow DOM before FAST elements connect.
  • The application must call enableHydration() from @microsoft/fast-element/hydration.js before hydrated elements connect.
  • If hydration is not enabled, or if a template is not hydratable, FAST uses client-side rendering for the element.

Use client rendering, or provide an application-owned DSD transform/polyfill, for browsers that meet the baseline runtime requirements but do not support Declarative Shadow DOM.