diff --git a/frontend/src/routes/layout.css b/frontend/src/routes/layout.css
index fd4912fe..85c41e67 100644
--- a/frontend/src/routes/layout.css
+++ b/frontend/src/routes/layout.css
@@ -153,7 +153,11 @@
 	--c-badge-unknown-text: #7a5a0a;
 	--c-badge-unknown-border: #f0ddb3;
 
-	/* Timeline density bars (issue #385) */
+	/* Timeline density bars (issue #385) — composited rgba(161,220,216,0.35)
+	   over --c-surface (#ffffff) ≈ #DEF3F1 → ~1.13:1 vs surface. Decorative
+	   idle fill in the same WCAG carve-out as --c-accent above; the meaningful
+	   selected/preview state uses --palette-mint (1.52:1 vs white). Resample
+	   with axe (tracked in #480) before tweaking the palette. */
 	--timeline-bar-idle: rgba(161, 220, 216, 0.35);
 	--timeline-bar-outside: var(--c-line);
 }