The example with x/d seems to be wrong because the compiler inserts a redundant <mrow> around the slash operator. Temml seems to render it better. (There is still spacing, unlike the LaTeX version, but honestly I prefer that.)

You are right. Getting rid of the stray <mrow> around '/' does make the spacing better. Also, today I learnt about Temml. It looks very interesting and I'll be trying it out. Thanks for this nice discussion!