Why might `setFocus()` not work when triggered from a custom JavaScript error handler that's attempting to target a PHP-generated selector?

The issue likely stems from timing and the client-server relationship. PHP generates the HTML *before* JavaScript executes. If your JavaScript error handler tries to `setFocus()` on an element identified by a PHP-generated selector *before* that element is fully rendered and available in the DOM, the element won't exist yet, resulting in 'is not defined' or similar errors. Ensure the JavaScript code that calls `setFocus()` runs *after* the DOM is fully loaded (e.g., using `DOMContentLoaded` event listener or placing the script tag at the end of the ``). Also, verify the selector itself is correct and hasn't been altered during rendering.