% !TeX encoding = utf8
\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{amsfonts}
\usepackage{xparse}
\usepackage{mathtools}

\newcommand\hi[1]{Hello #1!}

\ExplSyntaxOn
  % Convert a roman number into an arabic one
  \NewDocumentCommand \romantonum { m }
    { \int_from_roman:n { #1 } }
\ExplSyntaxOff

\begin{document}

\section{Highlight.js}
\hi{you}
This is \LaTeX\ syntax highlighted by \verb|highlight.js|.
You should look at section~\ref{sec:fermat} next.

Did you know that MDCLXI is \romantonum{MDCLXI}? % It's 1661.

\subsection{Fermat}\label{sec:fermat}
I have a wonderful proof that
\[
  a^n + b^n \neq c^n
\]
for \(a, b, c, d, n \in \mathbb{Z}_+\) with \(n > 2\).
Sadly, it is too large to fit in this code snippet.

\end{document}