diff --git a/chrome/test/data/pdf/accessibility/whitespace-expected.txt b/chrome/test/data/pdf/accessibility/whitespace-expected.txt
index 8d1c8b6..e69de29 100644
--- a/chrome/test/data/pdf/accessibility/whitespace-expected.txt
+++ b/chrome/test/data/pdf/accessibility/whitespace-expected.txt
@@ -1 +0,0 @@
- 
diff --git a/content/browser/accessibility/dump_accessibility_browsertest_base.cc b/content/browser/accessibility/dump_accessibility_browsertest_base.cc
index fb723a1..f6eed610 100644
--- a/content/browser/accessibility/dump_accessibility_browsertest_base.cc
+++ b/content/browser/accessibility/dump_accessibility_browsertest_base.cc
@@ -367,7 +367,6 @@
   }
 
   // Validate against the expectation file.
-  actual_lines.push_back(kMarkEndOfFile);
   bool matches_expectation = test_helper.ValidateAgainstExpectation(
       file_path, expected_file, actual_lines, *expected_lines);
   EXPECT_TRUE(matches_expectation);
diff --git a/content/public/test/dump_accessibility_test_helper.cc b/content/public/test/dump_accessibility_test_helper.cc
index ac1b83e..1b4aeab 100644
--- a/content/public/test/dump_accessibility_test_helper.cc
+++ b/content/public/test/dump_accessibility_test_helper.cc
@@ -19,9 +19,8 @@
 const char kCommentToken = '#';
 const char kMarkSkipFile[] = "#<skip";
 const char kSignalDiff[] = "*";
-}  // namespace
-
 const char kMarkEndOfFile[] = "<-- End-of-file -->";
+}  // namespace
 
 DumpAccessibilityTestHelper::DumpAccessibilityTestHelper(
     AccessibilityTestExpectationsLocator* test_locator)
@@ -80,10 +79,6 @@
       base::SplitString(expected_contents, "\n", base::KEEP_WHITESPACE,
                         base::SPLIT_WANT_NONEMPTY);
 
-  // Marking the end of the file with a line of text ensures that
-  // file length differences are found.
-  expected_lines.push_back(kMarkEndOfFile);
-
   return expected_lines;
 }
 
@@ -123,6 +118,11 @@
     diff += "------\n";
     diff += base::JoinString(actual_lines, "\n");
     diff += "\n";
+
+    // This is used by rebase_dump_accessibility_tree_test.py to signify
+    // the end of the file when parsing the actual output from remote logs.
+    diff += kMarkEndOfFile;
+    diff += "\n";
     LOG(ERROR) << "Diff:\n" << diff;
   } else {
     LOG(INFO) << "Test output matches expectations.";
@@ -163,6 +163,20 @@
     ++j;
   }
 
+  // Report a failure if there are additional expected lines or
+  // actual lines.
+  if (i < actual_lines_count) {
+    diff_lines.push_back(j);
+  } else {
+    while (j < expected_lines_count) {
+      if (expected_lines[j].size() > 0 &&
+          expected_lines[j][0] != kCommentToken) {
+        diff_lines.push_back(j);
+      }
+      j++;
+    }
+  }
+
   // Actual file has been fully checked.
   return diff_lines;
 }
diff --git a/content/public/test/dump_accessibility_test_helper.h b/content/public/test/dump_accessibility_test_helper.h
index a72b9c3..c4c9763 100644
--- a/content/public/test/dump_accessibility_test_helper.h
+++ b/content/public/test/dump_accessibility_test_helper.h
@@ -5,6 +5,7 @@
 #ifndef CONTENT_PUBLIC_TEST_DUMP_ACCESSIBILITY_TEST_HELPER_H_
 #define CONTENT_PUBLIC_TEST_DUMP_ACCESSIBILITY_TEST_HELPER_H_
 
+#include "base/gtest_prod_util.h"
 #include "base/optional.h"
 
 namespace base {
@@ -13,9 +14,6 @@
 
 namespace content {
 
-// Sentinal value to mark end of actual/expected results.
-extern const char kMarkEndOfFile[];
-
 class AccessibilityTestExpectationsLocator;
 
 // A helper class for writing accessibility tree dump tests.
@@ -46,6 +44,8 @@
       const std::vector<std::string>& expected_lines);
 
  private:
+  FRIEND_TEST_ALL_PREFIXES(DumpAccessibilityTestHelperTest, TestDiffLines);
+
   // Utility helper that does a comment-aware equality check.
   // Returns array of lines from expected file which are different.
   static std::vector<int> DiffLines(
diff --git a/content/public/test/dump_accessibility_test_helper_unittest.cc b/content/public/test/dump_accessibility_test_helper_unittest.cc
new file mode 100644
index 0000000..9a20e6f9
--- /dev/null
+++ b/content/public/test/dump_accessibility_test_helper_unittest.cc
@@ -0,0 +1,74 @@
+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "content/public/test/dump_accessibility_test_helper.h"
+
+#include "testing/gmock/include/gmock/gmock.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+using testing::ElementsAre;
+
+namespace content {
+
+TEST(DumpAccessibilityTestHelperTest, TestDiffLines) {
+  // Files with the same lines should have an empty diff.
+  EXPECT_THAT(DumpAccessibilityTestHelper::DiffLines({"broccoli", "turnip"},
+                                                     {"broccoli", "turnip"}),
+              ElementsAre());
+
+  // Empty files should have an empty diff.
+  EXPECT_THAT(DumpAccessibilityTestHelper::DiffLines({}, {}), ElementsAre());
+
+  // If one line differs, that line number should be returned as the diff.
+  EXPECT_THAT(DumpAccessibilityTestHelper::DiffLines({"broccoli", "turnip"},
+                                                     {"watermelon", "turnip"}),
+              ElementsAre(0));
+
+  EXPECT_THAT(DumpAccessibilityTestHelper::DiffLines({"broccoli", "turnip"},
+                                                     {"broccoli", "rutabaga"}),
+              ElementsAre(1));
+
+  // Multiple lines can differ.
+  EXPECT_THAT(DumpAccessibilityTestHelper::DiffLines(
+                  {"broccoli", "turnip", "carrot", "squash"},
+                  {"broccoli", "aspartame", "carrot", "toothbrush"}),
+              ElementsAre(1, 3));
+
+  // Blank lines in the expected file are allowed.
+  EXPECT_THAT(DumpAccessibilityTestHelper::DiffLines(
+                  {"", "broccoli", "turnip", "carrot", "", "squash"},
+                  {"broccoli", "turnip", "carrot", "squash"}),
+              ElementsAre());
+
+  // Comments in the expected file are allowed.
+  EXPECT_THAT(
+      DumpAccessibilityTestHelper::DiffLines(
+          {"#Vegetables:", "broccoli", "turnip", "carrot", "", "squash"},
+          {"broccoli", "turnip", "carrot", "squash"}),
+      ElementsAre());
+
+  // Comments or blank lines in the actual file are NOT allowed, that's a diff.
+  EXPECT_THAT(
+      DumpAccessibilityTestHelper::DiffLines(
+          {"broccoli", "turnip", "carrot", "squash"},
+          {"#vegetables", "broccoli", "turnip", "carrot", "", "squash"}),
+      ElementsAre(0, 1, 2, 3, 4));
+
+  // If the expected file has an extra line, that's a diff.
+  EXPECT_THAT(DumpAccessibilityTestHelper::DiffLines(
+                  {"broccoli", "turnip", "cow"}, {"broccoli", "turnip"}),
+              ElementsAre(2));
+
+  // If the actual file has an extra line, that's a diff.
+  EXPECT_THAT(DumpAccessibilityTestHelper::DiffLines(
+                  {"broccoli", "turnip"}, {"broccoli", "turnip", "horse"}),
+              ElementsAre(2));
+
+  // If the expected file has an extra blank line, that's okay.
+  EXPECT_THAT(DumpAccessibilityTestHelper::DiffLines({"broccoli", "turnip", ""},
+                                                     {"broccoli", "turnip"}),
+              ElementsAre());
+}
+
+}  // namespace content
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index 548ce84..f47de53e 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -1937,6 +1937,7 @@
     "../public/common/url_utils_unittest.cc",
     "../public/test/browser_task_environment_unittest.cc",
     "../public/test/devtools_permission_overrides_unittest.cc",
+    "../public/test/dump_accessibility_test_helper_unittest.cc",
     "../public/test/no_renderer_crashes_assertion_unittest.cc",
     "../public/test/permission_type_unittest.cc",
     "../public/test/referrer_unittest.cc",