diff --git a/src/browser/tests/window/window.html b/src/browser/tests/window/window.html
index 5d36e74d..8a024089 100644
--- a/src/browser/tests/window/window.html
+++ b/src/browser/tests/window/window.html
@@ -75,6 +75,16 @@
   testing.expectEqual('abc', atob('YWJj'));
   testing.expectEqual('0123456789', atob('MDEyMzQ1Njc4OQ=='));
   testing.expectEqual('The quick brown fox jumps over the lazy dog', atob('VGhlIHF1aWNrIGJyb3duIGZveCBqdW1wcyBvdmVyIHRoZSBsYXp5IGRvZw=='));
+
+  // atob must accept unpadded base64 (forgiving-base64 decode per HTML spec)
+  testing.expectEqual('a', atob('YQ'));      // 2 chars, len%4==2, needs '=='
+  testing.expectEqual('ab', atob('YWI'));    // 3 chars, len%4==3, needs '='
+  testing.expectEqual('ceil', atob('Y2VpbA'));  // 6 chars, len%4==2, needs '=='
+
+  // length % 4 == 1 must still throw
+  testing.expectError('Error: InvalidCharacterError', () => {
+    atob('Y');
+  });
 </script>
 
 <script id=btoa_atob_roundtrip>
diff --git a/src/browser/webapi/Window.zig b/src/browser/webapi/Window.zig
index 194ce397..c68d1f09 100644
--- a/src/browser/webapi/Window.zig
+++ b/src/browser/webapi/Window.zig
@@ -396,9 +396,28 @@ pub fn btoa(_: *const Window, input: []const u8, page: *Page) ![]const u8 {
 
 pub fn atob(_: *const Window, input: []const u8, page: *Page) ![]const u8 {
     const trimmed = std.mem.trim(u8, input, &std.ascii.whitespace);
-    const decoded_len = std.base64.standard.Decoder.calcSizeForSlice(trimmed) catch return error.InvalidCharacterError;
+    // Per HTML spec "forgiving-base64 decode" algorithm:
+    // https://infra.spec.whatwg.org/#forgiving-base64-decode
+    const padded: []const u8 = switch (trimmed.len % 4) {
+        1 => return error.InvalidCharacterError,
+        2 => blk: {
+            const buf = try page.call_arena.alloc(u8, trimmed.len + 2);
+            @memcpy(buf[0..trimmed.len], trimmed);
+            buf[trimmed.len] = '=';
+            buf[trimmed.len + 1] = '=';
+            break :blk buf;
+        },
+        3 => blk: {
+            const buf = try page.call_arena.alloc(u8, trimmed.len + 1);
+            @memcpy(buf[0..trimmed.len], trimmed);
+            buf[trimmed.len] = '=';
+            break :blk buf;
+        },
+        else => trimmed,
+    };
+    const decoded_len = std.base64.standard.Decoder.calcSizeForSlice(padded) catch return error.InvalidCharacterError;
     const decoded = try page.call_arena.alloc(u8, decoded_len);
-    std.base64.standard.Decoder.decode(decoded, trimmed) catch return error.InvalidCharacterError;
+    std.base64.standard.Decoder.decode(decoded, padded) catch return error.InvalidCharacterError;
     return decoded;
 }