Add required HTML attribute to required fields on PMPro Checkout Page.

add-required-html-attribute.php

<?php
/**
 * Add HTML5 attribute required to all required PMPro fields.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_add_required_html_attribute() {
	global $pmpro_pages;

	if ( is_page( $pmpro_pages['checkout'] ) || is_page( $pmpro_pages['billing'] ) || is_page( $pmpro_pages['member_profile_edit'] ) ) {
		?>
		<script>
			[...document.querySelectorAll(".pmpro_required")].forEach(element => { element.setAttribute("required", "required") });
			<?php if ( is_page( $pmpro_pages['member_profile_edit'] ) ) { ?>
			document.getElementById('user_email').setAttribute("required", "required");
			<?php } ?>
		</script>
		<?php
	}
}
add_action( 'wp_footer', 'my_pmpro_add_required_html_attribute', 20 );