[WordPress] @2ndkauboy has a nice hack to conditionally generate an IE legacy stylesheet with SASS and load it conditionally (http://kau-boys.de/1747) (post in German). This solution tweaks WordPress’s syntax for conditional comments accordingly.

wp-ie-conditional-comment-syntax-hack.php

<?php 
/**
 * Read first, otherwise the following will make no sense at all: 
 * http://kau-boys.de/1747
 * 
 * 
 * WordPress’s built-in conditional comment syntax allows 
 * for conditional comments like this:
 * 
 * <!--[if gt IE 8]>
 * <link rel='stylesheet' id='style-css' [etc] />
 * <!--<![endif]-->
 *
 * Result: both stylesheets are loaded equally which brings an 
 * unneccesary HTTP request and additioal overhead in style declarations.
 *
 * What we need is something like this:
 * 
 * <!--[if lte IE 8]>
 * <link rel='stylesheet' id='lte8-css' [etc] />
 * <![endif]-->
 * <!--[if gt IE 8]><!-->
 * <link rel='stylesheet' id='style-css' [etc] />
 * <!--<![endif]-->
 * 
 * Notice that tiny bit of comment in line 17: <!-->
 * It prevents IE8 and below from loading the default stylesheet at all!
 * Mission accomplished.
 * 
 */

/**
 * Enque stylesheets and scripts.
 */
add_action( 'wp_enqueue_scripts', 'gp130709_enqueue_stuff' );
function gp130709_enqueue_stuff() {
    global $wp_styles;
	
	/* IE legacy stylesheet. */
	wp_enqueue_style( 
		'lte8', 
		get_stylesheet_directory_uri() . '/assets/css/lte8.css'
	);
	$wp_styles->add_data( 'lte8', 'conditional', 'lte IE 8' );

	/* Theme stylesheet. */
	wp_enqueue_style(
		'style',
		get_stylesheet_directory_uri() . '/style.css'
	);
}


/**
 * Add custom syntax for conditional comments.
 */
add_filter( 'style_loader_tag', 'gp130709_style_loader_tag', 10, 2 );
function gp130709_style_loader_tag( $tag, $handle ) {
	
	/* Bail early if this is not the 
	 * default stylesheet.
	 */
	if( 'default' !== $handle )
		return $tag;
	
	$conditional  = '<!--[if gt IE 8]><!-->' . "\n";
	$conditional .= $tag;
	$conditional .= '<!--<![endif]-->' . "\n";
	
	return $conditional;
}


/**
 * Uncomment to remove filter, i.e. in a plugin or child theme.
 * 
 */
// if ( has_filter( 'style_loader_tag' ) )
// 	remove_filter( 'style_loader_tag', 'gp130709_style_loader_tag' );

Werde das bei Gelegenheit testen.