TYPO3 Core

I cannot confirm this: I am using custom CSS styles this way in 12.4.8. Have you cleared the cache (TYPO3 and browser)?

I can confirm this bug. It only happens with richtext editors inside flexform fields containing a dot, like "settings.text". TYPO3 then generates a the html with the identifier "#data_tt_content__6303__pi_flexform__data__sDEF__lDEF__settings.text__vDEF_ckeditor5" and a css file with rules like:

#data_tt_content__6303__pi_flexform__data__sDEF__lDEF__settings.text__vDEF_ckeditor5 .ck-content a {
  color: #0782C1;
  text-decoration: underline;
}

IDs with a "." are invalid and do not get their css styles correctly applied. Apparently something in FormEngine changed with an update, that now creates these IDs with a ".".

Ok, from what I see, that ids have always been generated with a ".", but the generated css, that now uses these ids, is new.