diff --git a/src/browser/ColorContrastCache.ts b/src/browser/ColorContrastCache.ts
index fdcd9d133199a6cd6ba9bea9606a02c03ad03b3d..ae8e06e77ba077aa5a47f0217dedaaaec0962a2f 100644
--- a/src/browser/ColorContrastCache.ts
+++ b/src/browser/ColorContrastCache.ts
@@ -7,11 +7,15 @@ import { IColorContrastCache } from './Types';
 import { IColor } from '../common/Types';
 import { TwoKeyMap } from '../common/MultiKeyMap';
 
+const CONTRAST_CACHE_MAX_ENTRIES = 4096;
+
 export class ColorContrastCache implements IColorContrastCache {
   private _color: TwoKeyMap</* bg */number, /* fg */number, IColor | null> = new TwoKeyMap();
   private _css: TwoKeyMap</* bg */number, /* fg */number, string | null> = new TwoKeyMap();
+  private _entryCount = 0;
 
   public setCss(bg: number, fg: number, value: string | null): void {
+    this._admitPair(bg, fg);
     this._css.set(bg, fg, value);
   }
 
@@ -20,6 +24,7 @@ export class ColorContrastCache implements IColorContrastCache {
   }
 
   public setColor(bg: number, fg: number, value: IColor | null): void {
+    this._admitPair(bg, fg);
     this._color.set(bg, fg, value);
   }
 
@@ -30,5 +35,19 @@ export class ColorContrastCache implements IColorContrastCache {
   public clear(): void {
     this._color.clear();
     this._css.clear();
+    this._entryCount = 0;
+  }
+
+  private _admitPair(bg: number, fg: number): void {
+    // Both setters admit, and the two maps only ever clear together, so either one
+    // already holding the pair means it was counted.
+    if (this._color.get(bg, fg) !== undefined || this._css.get(bg, fg) !== undefined) {
+      return;
+    }
+    // Color pairs outlive atlas pages, including cached misses and DOM-rendered colors.
+    if (this._entryCount >= CONTRAST_CACHE_MAX_ENTRIES) {
+      this.clear();
+    }
+    this._entryCount++;
   }
 }