(1) welcome, (2) how is this related to latex3? (3) I'd use \emph because then the emphasize also works in an italic context, \textit in an italic context is jut italic, thus you cannot see it.
– daleifDec 22 '17 at 10:49
\textit is physically formatting ("make it italic") and \emph is a logical formatting ("put emphasis on this text"). Now \emph uses \textit inside normal text, but this could be changed by a class or style. Also \emph inside \emph works will: the second one makes the text normal again to highlight it in the outer text. So, if you really want something italic use the first one, if you want to highlight something use \emph.
– Martin ScharrerDec 22 '17 at 11:02
I'm closing our question now, because it was already beeing asked. Please see the above linked question thread which will answer your question.
– Martin ScharrerDec 22 '17 at 11:04
@ZYX What you want is to mark something? Well, define a macro, and you will have clearer code and you can worry later if you want to change hte definition \newcommand*\markdefinition[1]{\emph{#1}} (finding a good name for the command is an art, you might like \dfntn{..} more than \markdefinition{..} or whatever.
– ManuelDec 22 '17 at 11:07
\emphbecause then the emphasize also works in an italic context,\textitin an italic context is jut italic, thus you cannot see it. – daleif Dec 22 '17 at 10:49\textitis physically formatting ("make it italic") and\emphis a logical formatting ("put emphasis on this text"). Now\emphuses\textitinside normal text, but this could be changed by a class or style. Also\emphinside\emphworks will: the second one makes the text normal again to highlight it in the outer text. So, if you really want something italic use the first one, if you want to highlight something use\emph. – Martin Scharrer Dec 22 '17 at 11:02\newcommand*\markdefinition[1]{\emph{#1}}(finding a good name for the command is an art, you might like\dfntn{..}more than\markdefinition{..}or whatever. – Manuel Dec 22 '17 at 11:07