Codebase Browser
chromium
Go to App

chromium/third_party/blink/web_tests/http/tests/html/resources/validation-bubble-oopif-long-message-iframe.html 

<!DOCTYPE html>
<input>
<script>
const input = document.querySelector('input');
input.setCustomValidity('foo'.repeat(100));
input.reportValidity();
</script>