Security issue and unnecessary code

Just some recommendations here. The headers should be added with send_headers… Especially the X-UA one. wp_headers is ran inside the send_headers() function before a cache check is done. So it won’t necessarily get sent if the page has already been statically cached…

Example including x-ua-compatible using send_headers

is_admin() || add_action('send_headers', function(){
    header('X-UA-Compatible: IE=edge,chrome=1');
});

As for the the html5 shim there’s a lot of unnecessary code. IE conditionals aren’t recognized above IE9 so this code essentially does nothing but add to the page load:

<!--[if !IE]>
	<script src="//html5shiv-printshiv.googlecode.com/svn/trunk/html5shiv-printshiv.js"></script>
	<![endif]-->
	<!--[if IE 13]>
	<script src="//html5shiv-printshiv.googlecode.com/svn/trunk/html5shiv-printshiv.js"></script>
	<![endif]-->
	<!--[if IE 12]>
	<script src="//html5shiv-printshiv.googlecode.com/svn/trunk/html5shiv-printshiv.js"></script>
	<![endif]-->
	<!--[if IE 11]>
	<script src="//html5shiv-printshiv.googlecode.com/svn/trunk/html5shiv-printshiv.js"></script>
	<![endif]-->
	<!--[if IE 10]>
	<script src="//html5shiv-printshiv.googlecode.com/svn/trunk/html5shiv-printshiv.js"></script>
	<![endif]-->

Also, it’s good for plugins to not use wp_head to include scripts. This way incase another plugin or theme includes this they will be able to dequeue the script. Plus wordpress has a built in way to do add the conditionals…

Example:

add_action('wp_enqueue_scripts', 'my_plugin_shim_fix', 99);
function my_plugin_shim_fix() {
        wp_enqueue_script( 'html5shiv', 'https://cdnjs.cloudflare.com/ajax/libs/html5shiv/3.7.3/html5shiv-printshiv.min.js', array( '' ), '3.7.2', false  );
        wp_scripts()->add_data( 'html5shiv', 'conditional', 'lte IE 9' );
}

Hope this helps 🙂

https://wordpress.org/plugins/wp-ie-enhancer-and-modernizer/