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<?php
// http://wordpress.stackexchange.com/questions/20876/less-css-enqueue-style-with-add-filter-to-change-rel-attribute

function enqueue_less_styles($tag, $handle) {
    global $wp_styles;
    $match_pattern = '/\.less$/U';
    if ( preg_match( $match_pattern, $wp_styles->registered[$handle]->src ) ) {
        $handle = $wp_styles->registered[$handle]->handle;
        $media = $wp_styles->registered[$handle]->args;
        $href = $wp_styles->registered[$handle]->src . '?ver=' . $wp_styles->registered[$handle]->ver;
        $rel = isset($wp_styles->registered[$handle]->extra['alt']) && $wp_styles->registered[$handle]->extra['alt'] ? 'alternate stylesheet' : 'stylesheet';
        $title = isset($wp_styles->registered[$handle]->extra['title']) ? "title='" . esc_attr( $wp_styles->registered[$handle]->extra['title'] ) . "'" : '';

        $tag = "<link rel='stylesheet' id='$handle' $title href='$href' type='text/less' media='$media' />";
    }
    return $tag;
}
add_filter( 'style_loader_tag', 'enqueue_less_styles', 5, 2);

<?php
// http://carlorizzante.com/development/quickly-use-less-wordpress-theme/

function load_LESS() {

	$path = get_stylesheet_directory_uri() . '/lesscss/';
	
	/* LESS stylesheet and less.js */
	$output = "\n"."<!-- Loading LESS stylesheet and less.js -->"."\n";
	$output .= '<link rel="stylesheet/less" type="text/css" href="' . $path . 'stylesheet.less' . '"/>' . "\n";
	$output .= '<script type="text/javascript" src="' . $path . 'less.js"></script>' ."\n";
	
	/* Auto refreshing the page when saving the stylesheet */
	$output .= "\n" . "<!-- Autorefresh for LESS.js -->" . "\n";
	$output .= '<script type="text/javascript">' . "\n";
	$output .= '  less.env = "development";' . "\n";
	$output .= '  less.watch();' . "\n";
	$output .= '</script>' . "\n\n";
	
	/* Printing on page */
	print $output;
}
if ( ! is_admin()) add_action( 'wp_head', 'load_LESS' );