TYPO3 Core

Both fluid_styled_content and css_styled_content include this in their lib.parseFunc:

lib.parseFunc {
[...]
    tags {
        [...]
        a {
            [...]
            typolink {
                [...]
                aTagParams.data = parameters:allParams
            }
        }
    }
}

which should, theoretically, make sure that if you e.g. write <a href="#" class="btn"> or similar in your CE, the class parameter is retained in the FE output.
Sadly, this is not the case and all parameters are dropped. The cause is that the typolink aTagParams parameter must be written with an uppercase A.