/** Properties an app can never allow, whatever its config. */
export declare const EXTENSION_DENYLIST: Set<string>;
/** Lowercase name (optionally `mso-`), not denylisted and not built in. */
export declare function isExtensionPropValid(name: string): boolean;
/** Strict value grammar: no functions except numeric colours, no escape characters. */
export declare function isExtensionValueValid(value: string): boolean;
/** tag → set of accepted extension property names. */
export type NormalizedAllowedStyles = Map<string, Set<string>>;
/** Validated tag → props map; warns once per dropped entry. */
export declare function normalizeEmailAllowedStyles(config: Record<string, string[]>): NormalizedAllowedStyles;
/** `"td: letter-spacing; p: text-indent"` → `{ td: ['letter-spacing'], p: ['text-indent'] }`. */
export declare function parseAllowedStylesAttr(raw: string): Record<string, string[]>;
/** Re-validates a stored `extraStyle` on write-back, where the per-app allowlist isn't visible. */
export declare function sanitizeExtraStyleForWrite(s: string): string;