This uses HTML5 form validation. This is created by the browser, and does not exist in the DOM. Therefore Selenium cannot see this.
You can access this using JavaScript. Here is a brief code sample:
JavascriptExecutor jsExecutor = (JavascriptExecutor) driver;
WebElement field = driver.findElement(By.tagName("input")); // your input box
if (!(Boolean) jsExecutor.executeScript("return arguments[0].validity.valid;", field)) {
return (String) jsExecutor.executeScript("return arguments[0].validationMessage;", field);
}
The entire API is documented.
element.validity.valid
Returns true if the element's value has no validity problems; false otherwise.
So this popup is displayed if this returns false
, but only after clicking Submit on the form.