italicization with \textit does not work

Question

When I compile the following, ABC is not italicized.

\documentclass[a4paper,12pt]{amsart}
\begin{document}
\textsf{\textbf{\textit{ABC}}}
\end{document} 

Answer 1

You'll notice the following warning in your .log file:

LaTeX Font Warning: Font shape `OT1/cmss/bx/it' undefined
(Font)              using `OT1/cmss/bx/n' instead on input line 4.

It suggests that under the OT1 font encoding, there is no bold bx *italic** it version within the sans serif font. So, it substitutes it just for a normal bold, hence the output

You need a font that includes this typeface. lmodern does:

\documentclass{amsart}
\usepackage{lmodern}
\begin{document}
\textsf{\textbf{\textit{ABC}}}
\end{document}