/**
 * Injects a CSS string into a ShadowRoot as a `<style>` element.
 *
 * Idempotent: reuses the element with the given id and only updates it when the
 * CSS changed. Throws when the target is not a ShadowRoot, so styles can never
 * leak outside the shadow tree.
 * @param {ShadowRoot} shadowRoot - The shadow root to inject into (required for isolation).
 * @param {string} cssText - The CSS to inject.
 * @param {string} styleElementId - Stable id for the injected `<style>` element.
 * @returns {void} Nothing.
 * @throws {Error} When `shadowRoot` is not a ShadowRoot instance.
 */
export declare const injectShadowStyles: (shadowRoot: ShadowRoot, cssText: string, styleElementId: string) => void;
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