diff --git a/dom/html/ConstraintValidation.cpp b/dom/html/ConstraintValidation.cpp
index 1d256a309287..4d202b5fbfc8 100644
--- a/dom/html/ConstraintValidation.cpp
+++ b/dom/html/ConstraintValidation.cpp
@@ -9,6 +9,7 @@
 #include "mozilla/ErrorResult.h"
 #include "nsAString.h"
 #include "nsIContent.h"
+#include "nsContentUtils.h"
 
 namespace mozilla::dom {
 
@@ -59,7 +60,9 @@ bool ConstraintValidation::CheckValidity() {
 ConstraintValidation::ConstraintValidation() = default;
 
 void ConstraintValidation::SetCustomValidity(const nsAString& aError) {
-  mCustomValidity.Assign(aError);
+  nsAutoString error(aError);
+  nsContentUtils::PlatformToDOMLineBreaks(error);
+  mCustomValidity.Assign(error);
   SetValidityState(VALIDITY_STATE_CUSTOM_ERROR, !mCustomValidity.IsEmpty());
 }
 
diff --git a/testing/web-platform/tests/html/semantics/forms/setCustomValidity-normalize-newlines.html b/testing/web-platform/tests/html/semantics/forms/setCustomValidity-normalize-newlines.html
new file mode 100644
index 000000000000..3eb8c17e8f5b
--- /dev/null
+++ b/testing/web-platform/tests/html/semantics/forms/setCustomValidity-normalize-newlines.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<html>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<form>
+  <input id="email" required>
+</form>
+<script>
+test(function() {
+  const input = document.getElementById("email");
+  input.setCustomValidity("First line\rSecond line\r\nThird line\nFourth line");
+  assert_equals(input.validationMessage, "First line\nSecond line\nThird line\nFourth line");
+}, "setCustomValidity should normalize newlines from the given error message");
+</script>
+</html>
