diff --git a/src/styles/signin.css b/src/styles/signin.css
index 5beac15..2da8d81 100644
--- a/src/styles/signin.css
+++ b/src/styles/signin.css
@@ -100,5 +100,4 @@
 }
 .signin-field input::placeholder { color: var(--ink-3); }
 .signin-form .signin-submit { margin-top: 18px; width: 100%; height: 54px; }
-.signin-form .signin-submit:disabled { opacity: .5; cursor: not-allowed; }
 .signin-error { margin: 0; }
diff --git a/tests/disabled-opacity.test.ts b/tests/disabled-opacity.test.ts
new file mode 100644
index 0000000..6d21d36
--- /dev/null
+++ b/tests/disabled-opacity.test.ts
@@ -0,0 +1,52 @@
+import { describe, expect, it } from "vitest";
+import { readFileSync, readdirSync, statSync } from "node:fs";
+import path from "node:path";
+
+/** Every .css file under src/ (src/styles, src/app/globals.css, and any other). */
+function cssFiles(dir: string): string[] {
+  return readdirSync(dir).flatMap((name) => {
+    const full = path.join(dir, name);
+    if (statSync(full).isDirectory()) return cssFiles(full);
+    return full.endsWith(".css") ? [full] : [];
+  });
+}
+
+/** Rules whose selector targets a disabled state and whose opacity is below 1. */
+function fadedDisabledRules(source: string): string[] {
+  const text = source.replace(/\/\*[\s\S]*?\*\//g, "");
+  const found: string[] = [];
+  // Innermost blocks only: the selector text runs back to the previous brace,
+  // so rules nested inside @media or @supports are scanned too.
+  for (const m of text.matchAll(/([^{}]+)\{([^{}]*)\}/g)) {
+    const selector = m[1].trim();
+    if (!/:disabled|\[disabled\]|\[aria-disabled(="true")?\]/.test(selector)) continue;
+    for (const o of m[2].matchAll(/(?:^|;|\s)opacity\s*:\s*([\d.]+)(%?)/g)) {
+      const value = parseFloat(o[1]) / (o[2] ? 100 : 1);
+      if (value < 1) found.push(`${selector} { opacity: ${o[1]}${o[2]} }`);
+    }
+  }
+  return found;
+}
+
+describe("disabled buttons are never faded", () => {
+  const root = path.resolve(__dirname, "../src");
+  const files = cssFiles(root);
+
+  it("finds the stylesheets, globals.css included", () => {
+    expect(files.some((f) => f.endsWith(path.join("app", "globals.css")))).toBe(true);
+    expect(files.some((f) => f.endsWith(path.join("styles", "signin.css")))).toBe(true);
+  });
+
+  it("the scanner catches a faded disabled rule", () => {
+    expect(fadedDisabledRules(".a:disabled { opacity: .5; }")).toHaveLength(1);
+    expect(fadedDisabledRules("@media (x) { .a[disabled] { color: red; opacity: 40% } }")).toHaveLength(1);
+    expect(fadedDisabledRules(".a:disabled { opacity: 1; }")).toHaveLength(0);
+    expect(fadedDisabledRules(".a:hover { opacity: .5; }")).toHaveLength(0);
+  });
+
+  for (const file of files) {
+    it(`${path.relative(root, file)} has no :disabled rule with opacity below 1`, () => {
+      expect(fadedDisabledRules(readFileSync(file, "utf8"))).toEqual([]);
+    });
+  }
+});