I have just discovered that MathJax has an extension that allows you to write chemical equations! In DT3 you don’t need to do anything else to enable this (as long as MathJax has been turned on in Preferences).
You just need to include
\ce after the first
ce = “chemical equation”) and enclose the equation in braces. Here are some random equations I typed out as examples:
The guide is here: Manual for: mhchem for MathJax, mhchem for KaTeX