Add class to posts navigation links for WordPress

post-navigation-filter.php

/**
 * Filter to add class to next/previous navigation links
 */
function posts_link_attributes() {
    return 'class="mdlwp-nav__button"';
}
add_filter('next_posts_link_attributes', 'posts_link_attributes');
add_filter('previous_posts_link_attributes', 'posts_link_attributes');
 
function post_link_attributes($output) {
    $class = 'class="mdlwp-nav__button"';
    return str_replace('<a href=', '<a '.$class.' href=', $output);
}
add_filter('next_post_link', 'post_link_attributes');
add_filter('previous_post_link', 'post_link_attributes');