TYPO3 Core

The u tag is stripped out on the way to the database by the default processing options because it is deprecated. You will need to add it to RTE.default.proc.allowTags in Page TSConfig:

RTE.default.proc {
allowTags := addToList(u)
entryHTMLparser_db.allowTags := addToList(u)
}

Then you still have to remove it from "removeTags" where it is
explicitly mentioned:

RTE.default {
removeTags := removeFromList(u)
proc.entryHTMLparser_db.removeTags := removeFromList(u)
}