Function layout_2020::flow::justify_self_alignment

fn justify_self_alignment(
    containing_block: &ContainingBlock<'_>,
    free_space: Au,
) -> Au

This is supposed to handle ‘justify-self’, but no browser supports it on block boxes. Instead, <center> and <div align> are implemented via internal ‘text-align’ values. The provided free space should already take margins into account. In particular, it should be zero if there is an auto margin. https://drafts.csswg.org/css-align/#justify-block