From the Helix docs, I found that if I passed the :class prop into my components, it would automagically translate them to :className

https://github.com/lilactown/helix/blob/master/docs/creating-elements.md

See “Other special props:” subsection

In practice, it was working until I integrated with a library that clones an element or does some more complex interaction. Then, :class was not properly replaced to :className until a second render, and I just decided to use :className as I’m anyways used to that on React (js) and don’t mind.

I suppose, to directly address your question: I don’t know if Helix transforms class to className in those scenarios (web components), or if it should.