Platform Baseline
CHROMIUM_BASELINE_TARGET: Chromium 133+
Symbiote UI treats Chromium 133+ as the browser-platform target for target-schema browser smoke and feature planning. Runtime code must still feature-detect browser surfaces before use; this target is a planning baseline, not a user-agent gate.
Rows marked shimmed or capability-detected are the allowed R-UI28 exception
classes. They must be re-graded when the underlying API reaches stable Chromium
and whenever CHROMIUM_BASELINE_TARGET changes.
Feature Ledger #
| Feature | Status | Shim | Spec consumers | Capability gate |
|---|---|---|---|---|
| URLPattern | baseline | - | Route schema, route matcher, server-plane URL contracts | globalThis.URLPattern |
| Element.moveBefore / connectedMoveCallback | baseline | - | Framework handoff, state-preserving shell moves, WebMCP target continuity | Element.prototype.moveBefore; connectedMoveCallback tracked with the framework backlog |
| Declarative Shadow DOM | baseline | - | SSR process HTML, stream rendering, root shadow hydration | HTMLTemplateElement.prototype.shadowRootMode |
| adoptedStyleSheets / rootStyles delivery | baseline | - | SSR rootStyles, cascade themes, constructable stylesheet delivery | Document.prototype.adoptedStyleSheets, ShadowRoot.prototype.adoptedStyleSheets, CSSStyleSheet.prototype.replaceSync |
| view-transition-name custom idents | baseline | - | Workspace address serialization, panel transition targets, route animations | CSS.supports("view-transition-name", "workspace-panel-main") |
| document.modelContext | shimmed | symbiote-webmcp-shim | WebMCP tool registration, tour context, agent-visible component actions | document.modelContext; otherwise install symbiote-webmcp-shim |
Source Notes #
- URLPattern is part of the current browser baseline and remains feature-detected because route matching must also run in non-browser tests.
- Declarative Shadow DOM and constructable stylesheets are required for SSR and rootStyles delivery paths, but hosts still own fallback rendering where a target browser does not expose the feature.
document.modelContextis intentionally shimmed until the WebMCP surface is stable in Chromium; this row is the R-UI28 exception that must be re-graded first.