Find Element Call's root by attribute, not by being the body

Six selectors named `body` directly — the gradient backdrop and the
platform font overrides in index.css, and the iOS adjustments in
AppBar.module.css and Modal.module.css — so they only applied when Element
Call owned the page. A host mounting it into a container would have got an
interface decorated correctly and styled incorrectly, with nothing to show
that anything was wrong.

Mark the root element with data-element-call-root and match on that
instead. Scoping this way keeps the selectors more specific than they were,
rather than less: widening them to a bare [data-platform=…] would have
dropped specificity from (0,1,1) to (0,1,0) and changed which rules win.

The platform attribute moves with them, from the initializer's write onto
document.body to a layout effect on the root, alongside the theme — so it
still lands before anything is painted.

No visual change while Element Call owns the page: the root is the body,
which now carries the attribute, so every rewritten selector matches the
element it always did.

src/AppBar.module.css

@@ -131,7 +131,7 @@
   }
 }
 
-body[data-platform="ios"] {
+[data-element-call-root][data-platform="ios"] {
   .bar > header {
     grid-template-rows: minmax(var(--cpd-space-11x), auto) var(--cpd-space-4x);
     grid-template-areas: "primaryButton title secondaryButton";

src/Modal.module.css

@@ -48,7 +48,7 @@ Please see LICENSE in the repository root for full details.
   --handle-inset-block-end: var(--cpd-space-4x);
 }
 
-body[data-platform="ios"] .drawer {
+[data-element-call-root][data-platform="ios"] .drawer {
   --border-radius: 10px;
   --handle-block-size: 5px;
   --handle-inline-size: 36px;

src/RootElementContext.ts

@@ -16,17 +16,18 @@ import { createContext, use } from "react";
  * that when embedded in a host application it becomes the container the host
  * mounted it into, so that Element Call does not reach outside its own subtree.
  *
- * That intent is not yet achievable: several selectors still name `body`
- * directly — `body[data-background="gradient"]` and `body[data-platform=…]` in
- * `index.css`, and `body[data-platform="ios"]` in `AppBar.module.css` and
- * `Modal.module.css` — and `Initializer.initBeforeReact` writes
- * `data-platform` straight onto the body. So anything other than the body will
- * be decorated correctly and styled incorrectly, silently. Until those are
- * scoped, treat this as preparation rather than a working seam.
+ * The stylesheets find this element by its `data-element-call-root` attribute,
+ * which {@link useTheme} sets along with the platform and theme, so they no
+ * longer depend on it being the body.
+ *
+ * What remains body-specific is the standalone page's own furniture: the
+ * `body` rule in `index.css` still sets the page background and margin, and
+ * `index.html` starts the body hidden with `no-theme` until the theme lands.
+ * Neither applies when a host mounts Element Call into a container of its own.
  */
 // No provider is exported yet: nothing supplies a root element, so every
-// consumer falls back to the document body. M1 adds one along with the
-// component that mounts Element Call into a container.
+// consumer falls back to the document body. One arrives with the entry point
+// that mounts Element Call into a container.
 const RootElementContext = createContext<HTMLElement | null>(null);
 
 /**

src/index.css

@@ -75,7 +75,7 @@ body {
 }
 
 @media (min-height: 330px) {
-  body[data-background="gradient"]::before {
+  [data-element-call-root][data-background="gradient"]::before {
     content: "";
     position: fixed;
     /* Chromium abruptly fades our images to fully transparent at the edge of
@@ -88,7 +88,7 @@ body {
     background-repeat: no-repeat;
   }
 
-  body[data-background="gradient"][data-platform="desktop"]::before {
+  [data-element-call-root][data-background="gradient"][data-platform="desktop"]::before {
     background-image: url("graphics/desktop-gradient.png");
     background-size: max(1440px, 100vw) max(1440px, 100vh);
     background-position: center;
@@ -126,11 +126,11 @@ body,
 /* On Android and iOS, prefer native system fonts. The global.css file of
 Compound Web is where these variables ultimately get consumed to set the page's
 font-family. */
-body[data-platform="android"] {
+[data-element-call-root][data-platform="android"] {
   --cpd-font-family-sans: "Roboto", "Noto", "Inter", sans-serif;
 }
 
-body[data-platform="ios"] {
+[data-element-call-root][data-platform="ios"] {
   --cpd-font-family-sans:
     -apple-system, BlinkMacSystemFont, "Inter", sans-serif;
 }

src/initializer.tsx

@@ -30,7 +30,6 @@ import {
 import { getUrlParams } from "./UrlParams";
 import { Config } from "./config/Config";
 import { seedSettingsFromConfig } from "./settings/settings";
-import { platform } from "./Platform";
 import { isFailure } from "./utils/fetch";
 import { initializeWidget, type WidgetHelpers } from "./widget";
 import { enableExtendedLivekitLogs } from "./settings/settings.ts";
@@ -229,9 +228,6 @@ export class Initializer {
       );
     }
 
-    // Add the platform to the DOM, so CSS can query it
-    document.body.setAttribute("data-platform", platform);
-
     // livekit logging configuration
     setLKLogExtension((level, msg, context) => {
       // we pass a synthetic logger name of "livekit" to the rageshake to make it easier to read

src/useTheme.test.ts

@@ -19,6 +19,7 @@ import {
 } from "vitest";
 
 import { useTheme } from "./useTheme";
+import { platform } from "./Platform";
 import { useUrlParams } from "./UrlParams";
 import {
   type HostBridge,
@@ -88,6 +89,20 @@ describe("useTheme", () => {
     expect(originalClassList.add).not.toHaveBeenCalled();
   });
 
+  test("marks the element as Element Call's root, for the stylesheets", () => {
+    renderHook(() => useTheme(), { wrapper });
+
+    // The stylesheets find the root by this rather than by naming `body`, so
+    // that they still apply when Element Call is mounted into a container
+    expect(document.body.hasAttribute("data-element-call-root")).toBe(true);
+  });
+
+  test("records the platform on the root, for the stylesheets", () => {
+    renderHook(() => useTheme(), { wrapper });
+
+    expect(document.body.getAttribute("data-platform")).toBe(platform);
+  });
+
   test("theme changes in response to host requests", () => {
     renderHook(() => useTheme(), { wrapper });
 

src/useTheme.ts

@@ -9,6 +9,7 @@ import { useEffect, useLayoutEffect, useRef, useState } from "react";
 
 import { useUrlParams } from "./UrlParams";
 import { useRootElement } from "./RootElementContext";
+import { platform } from "./Platform";
 import { useHostBridge } from "./HostBridge";
 
 export const useTheme = (): void => {
@@ -28,6 +29,14 @@ export const useTheme = (): void => {
     return (): void => subscription.unsubscribe();
   }, [hostBridge]);
 
+  // Mark the element as Element Call's root and record the platform on it, so
+  // that the stylesheets can find both without naming `body`. A layout effect,
+  // like the theme below, so that it lands before anything is painted.
+  useLayoutEffect(() => {
+    rootElement.setAttribute("data-element-call-root", "");
+    rootElement.setAttribute("data-platform", platform);
+  }, [rootElement]);
+
   useLayoutEffect(() => {
     // If no theme has been explicitly requested we default to dark
     const theme = requestedTheme?.includes("light") ? "light" : "dark";