At what point in the parsing/rendering process do HTML elements get assigned a box model?

Question

Is it during the construction of the CSSOM (where possibly default padding/border/margin is applied to all nodes)?

Answer 1

An element's box is (re)calculated in what is called the "update layout" phase, which is currently specified* as happening (at least) inside the next "update the rendering" step of the event-loop, at sub-step 14:

14. For each fully active Document doc in docs:

• Recalculate styles and update layout for doc

but it can also be forced synchronously when calling some getters.

*_{Though this is a quite recent addition to the specs and not all browsers do follow it exactly, for instance I know from experience that to do this layout update Safari will not wait for the next "painting frame", but just some idle time.}