\documentclass{article}
\usepackage{latexml}
\begin{document}
% explicitly marked up application of three functions, each accepting a bare argument (= without fences)
\makeatletter
\iflatexml\else\gdef\@APPLYFUNCTION{}\fi
$f \@APPLYFUNCTION g \@APPLYFUNCTION h \@APPLYFUNCTION x$
% trigonometry bare arguments, tend to be parsed through a different path and need their own test
$\sin xy$
$\sin \pi\times x\cos 2\pi\times y$
% partial bare arguments surrounded preceded by multiplication
% 
$\times \sin xyz$
\end{document}