It appears the “demo-mode” of the TYPO3 system-extension RTE-HTMLArea is broken. One may believe so because quite a bunch of tags which are not only valid but also offered (GUI-wise) will be HCSed. The reason is in the oversight to provide a acompanying TS-config for the “demo-mode” Page-TS setup. As not everyone may have the ability to step and debug through the code I provide the TS-config you should use for the “demo-mode” here:

lib.parseFunc_RTE {
    removeTags := removeFromList(dl, dt, dd, cite)
    allowTags := addToList(dl, dt, dd, cite)
    denyTags := removeFromList(dl, dt, dd, cite)

    nonTypoTagStdWrap.encapsLines.addAttributes >
    externalBlocks := addToList(dl)

    externalBlocks.blockquote.callRecursive.tagStdWrap.HTMLparser.tags.blockquote.overrideAttribs >
    externalBlocks.dl < .ol
}