QUESTION How to deliver assets from sub-domain

[Lucianp]

I am trying to move my site assets (css, images,js) to a sub-domain to improve page load speed.
the site is small and local therefore unnecessary to use a CDN.

I set up a sub-domain static.example.com with folders css, img and js.

I changed my link from 

Code:

<link rel="stylesheet" href="<?php get_theme_url(); ?>/css/styles.css">

to 

Code:

<link rel="stylesheet" href="http://static.example.com/css/styles.css">

but the pages are not seeing the css.

clicking on the link from the source code shows the CSS file, so the link is good, so what am I missing?

thanks

[Lucianp]

found the problem, I was linking http sub-domain to https site.