Add classes to links generated by next_ & previous_posts_link.

README.md

Next/Prev Link Classes

WordPress Snippets

ccd-next-prev-link-class.php

<?php

// Single class for both links

function ccd_posts_link_attributes() {
    return 'class="next-prev-post"';
}

add_filter( 'next_posts_link_attributes', 'ccd_posts_link_attributes' );
add_filter( 'previous_posts_link_attributes', 'ccd_posts_link_attributes' );

?>

ccd-next-prev-link-classes.php

<?php

// Class for each links

function ccd_posts_link_attributes_2() {
    return 'class="next-post"';
}

function ccd_posts_link_attributes_1() {
    return 'class="prev-post"';
}

add_filter( 'next_posts_link_attributes', 'ccd_posts_link_attributes_1' );
add_filter( 'previous_posts_link_attributes', 'ccd_posts_link_attributes_2' );

?>