diff --git a/web/canvas.ts b/web/canvas.ts
index bf2ff5d..c89e746 100644
--- a/web/canvas.ts
+++ b/web/canvas.ts
@@ -512,6 +512,37 @@ export function paintMeasure(document: Document): () => void {
     // lines against the edges would be noise claiming to be information.
     if (box.width >= (document.documentElement.clientWidth || 0) - 1) return;
 
+    /*
+      The margins THEMSELVES, not only the lines that bound them.
+
+      Two hairlines say where the measure ends and leave the reader to notice that the space outside is
+      empty. Tinting it says what that space IS: room the page is not using, which is the question
+      somebody widening a gutter or judging a desktop layout is actually asking. On a narrow frame there
+      is nothing to tint and the block below returns before drawing any of it.
+    */
+    for (const [side, from, width] of [
+      ['left', 0, box.left],
+      ['right', box.right, (document.documentElement.clientWidth || 0) - box.right],
+    ] as const) {
+      if (width <= 0) continue;
+      const margin = document.createElement('div');
+      margin.setAttribute(MARK, `measure-margin-${side}`);
+      style(margin, [
+        'position:fixed',
+        'top:0',
+        'bottom:0',
+        `left:${Math.round(from)}px`,
+        `width:${Math.round(width)}px`,
+        // Faint, because this is the part of the screen the page does NOT occupy: it has to read as
+        // absence. Anything stronger competes with the content for the eye.
+        `background:color-mix(in srgb, ${ACCENT} 7%, transparent)`,
+        'pointer-events:none',
+        'z-index:2147482999',
+      ]);
+      document.body.append(margin);
+      guides.push(margin);
+    }
+
     for (const [side, x] of [
       ['left', box.left],
       ['right', box.right],