/**
 * Copies @font-face rules out of a stylesheet and into document.head.
 *
 * The laboratory renders inside a shadow root, and browsers resolve font faces
 * against the document's font set rather than a shadow tree's, so a @font-face
 * declared in the shadow style is ignored. Monaco's codicon glyphs (the folding
 * chevrons) render as empty squares without this.
 *
 * Idempotent across component instances, StrictMode double mounts, and multiple
 * copies of the bundle on one page.
 */
export declare function ensureDocumentFontFaces(css: string): void;
/** Test seam: resets the module-level guard. */
export declare function resetDocumentFontFacesForTests(): void;