Underline and strike in RTE?
Do you miss the underline or even the strike option in the Rich Text Editor (RTE)? Yeah, me too, although the <u>,<s> and <strike> tags have been deprecated since HTML4 in favor of css (use the text-decoration style property instead).
To enable the underline and strike options, put this in TSConfig of the root page:
# RTE config
#lib.parseFunc_RTE.allowTags = script,object,embed,param,b,i,u,a,img,br,div,center,pre,font,hr,sub,sup,p,strong,em,li,ul,ol,blockquote,strike,span,h1,h2,h3
RTE.default.defaultLinkTarget = _self
# activate underline option in RTE
RTE.default.proc.entryHTMLparser_db.removeTags := removeFromList(u)
RTE.default.FE.proc < RTE.default.proc
RTE.default.removeTags := removeFromList(u)
RTE.default.showButtons := addToList(underline)
RTE.default.FE < RTE.default
RTE.default {
removeTags = center, o:p, sdfield, strike
proc.entryHTMLparser_db.removeTags = center, o:p, sdfield, strike
# make sure the website css is used in the RTE:
contentCSS = fileadmin/templates/css/styles.css
}

Kind reminder! This tutorial has been tested with and updated for TYPO3 v10. Please keep in mind that the tutorial has once been set up for TYPO3 v6. It follows some deprecated conventions. If so, this will be mentioned in the tutorial.