The TSConfig option "proc.allowAttributes" is missing in the list on https://docs.typo3.org/m/typo3/reference-tsconfig/main/en-us/PageTsconfig/Rte.html#htmlparser-db The functionality is contained in `typo3/sysext/core/Classes/Html/RteHtmlParser.php` of the core. Got introduced a long time ago in https://review.typo3.org/50919