Allow only html doctype

Author: tosmolka

dist/purify.cjs.js

@@ -12,4 +12,4 @@ export const IS_SCRIPT_OR_DATA = seal(/^(?:\w+script|data):/i);
 export const ATTR_WHITESPACE = seal(
   /[\u0000-\u0020\u00A0\u1680\u180E\u2000-\u2029\u205F\u3000]/g // eslint-disable-line no-control-regex
 );
-export const DOCTYPE_NAME = seal(/^[a-z_.:]+$/i);
+export const DOCTYPE_NAME = seal(/^html$/i);