Function box_model 

pub fn box_model(
    style: &StyleRefinement,
    size: Size<Pixels>,
    rem_size: Pixels,
) -> BoxModel

Resolve the box model for an element, given the bounds it was laid out at.

rem_size is what turns rem-based style into the pixel numbers the diagram prints; percentage values have no parent to resolve against here and read as zero, exactly as Safari shows them when it cannot compute one.