The header section of a PhoenixPaper.Table (pp_table_head/1) — renders a <thead> containing PhoenixPaper.TableRows of variant="head" PhoenixPaper.TableCells. See PhoenixPaper.Table's moduledoc for a full example.

The border under the header lives here (on every descendant <th>, not on the <thead> element itself — a border-bottom on <thead> doesn't render reliably across browsers once border-collapse is set on the table, but one on each <th> does).