Defer Loading CSS in WordPress to Prevent Render-Blocking

functions.php

<?php
// Defer Loading CSS in WordPress to Prevent Render-Blocking
function custom_defer_css( $tag, $handle ) {
    $css_to_defer = array( 'style-handle-one', 'style-handle-two' );

    if ( in_array( $handle, $css_to_defer ) ) {
        return str_replace( "media='all'", "media='print' onload=\"this.media='all';\"", $tag );
    }
    
    return $tag;
}
add_filter( 'style_loader_tag', 'custom_defer_css', 10, 2 );