You can find the styles for the CSS animations in the header of each page. Animations where not possible entirely possible without using some JavaScript. For example each link inside the navigation requires a different animation delay. It would of been possible to use only CSS for this but you may run into trouble if you add more links, this way your navigation is future proof.
You can add as many links as you want in the navigation. The navigation will resize accordingly and the CSS will be generated automatically using JavaScript.
HTML
This is the HTML for the entire page.You have to note a few important things. The navigation icon has the 'mobile-inner-header-icon-out' class attached to it, this class is important for the animation of the icon. Another important thing is that there are two 'span' divs inside the header icon. Those will be the lines that animate when you click on them.
CSS
There are other CSS styles that are related to '.mobile-inner-nav a' which you can find inside 'main.css' file. You will find the styles that animate the link embedded inside this document.You can see that '.mobile-inner-nav a' has the animations 'returnToNormal' appointed to it.