Flow .section-items into CSS multi-column at >= 900px viewports: 2 cols at 900px, 3 at 1300px, 4 at 1800px, 5 at 2400px. Keep each .item-group and .item-dual together in a single column via break-inside: avoid so a group's label never separates from its items. Tighten row padding (0.35em → 0.22em) and body padding so the page reads as a denser index. Section headers get an underline separator to stay distinct now that the adjacent content has less vertical air. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>