Localhost - Using a localhost server is extremely vital in any HTML template, as it boosts the page load. For this reason, any template or website might work slowly or may not even work at all without localhost. Like in the case of the Master sliders and the Revolution Sliders. Opening Docs UI Kit in localhost server should not give you any problems.
In computer networking, localhost is a hostname that means this computer. It is used to access the network services that are running on the host via its loopback network interface. Using the loopback interface bypasses any local network interface hardware.
The local loopback mechanism is useful for testing software during development, independently of any networking configurations. For example, if a computer has been configured to provide a website, directing a locally running web browser to http://localhost may display its home page.
<!-- CSS Implementing Plugins -->
<link rel="stylesheet" href="assets/vendor/font-awesome/css/fontawesome-all.min.css">
<!-- CSS Docs UI Kit Template -->
<link rel="stylesheet" href="assets/css/theme.css">
Just so you know, Ajax also does not work without a localhost server. The whole point of Ajax request is to fetch data from one of these computers, which has to be listening in for specific network packages.