Here's a simple latex-based example.
---
header-includes:
- \usepackage[most]{tcolorbox}
- \definecolor{light-yellow}{rgb}{1, 0.95, 0.7}
- \newtcolorbox{myquote}{colback=light-yellow,grow to right by=-10mm,grow to left by=-10mm, boxrule=0pt,boxsep=0pt,breakable}
- \newcommand{\todo}[1]{\begin{myquote} \textbf{TODO:} \emph{#1} \end{myquote}}
---
blah blah
\todo{something}
blah
Unfortunately because this is latex, you can no-longer include markdown inside the TODO box (which is not a huge problem, usually), and it won't work when converting to formats other than PDF (e.g. html).