Users would like to add custom classes to links in RTE link wizard. I don't see any documentation about this feature.
On TYPO3 7 you have to add the followint Page TSConfig:
RTE.classesAnchor.myown-link {
class = myown-link
type = page
titleText = My Own Link
altText = My Own Link
}
RTE.default.buttons.link.properties.class.allowedClasses := addToList(myown-link)
RTE.default.proc.allowedClasses := addToList(myown-link)
See the documentation here and this answer, too