OpenReview uses the open source library MathJax to render TeX content, and there are some key differences between it and other systems. First and foremost, the TeX input processor implements only the math-mode macros of TeX and LaTeX, not the text-mode macros. So, for example, MathJax does not implement \emph or \begin{enumerate}...\end{enumerate} or other text-mode macros or environments. You must use Markdown (if enabled) to handle such formatting tasks.
Keep in mind that your mathematics is part of an HTML document, so you need to be aware of the special characters used by HTML as part of its markup. There cannot be HTML tags within the math delimiters as TeX-formatted math does not include HTML tags. Make sure to add spaces around any < or > symbols to ensure they are not treated as open tags.