I was under the impression that
\d
was causing errors in the compile (I know it is redefined somewhere in the TeX4ht process) However, this seems to have compiled fine, but in the tikz image (!) there's a \d that's rendered as a dot. (between lines 65 and 75)
https://set-p-dsb-zomercursus-latest.cloud-ext.icts.kuleuven.be/wimcalculusluadeployparallel/calculus2/trigonometricSubstitution/digInTrigonometricSubstitution