Kanso is a set of semantic flat buttons written in CSS and HTML. SASS and LESS source code available. It currently supports IE8+ and modern browsers. It also can work with Font Awesome Icons to create icon buttons.
How to use it:
Include Kanso.css in the head section of your web page.
<link rel="stylesheet" href="css/kanso.css">
Create buttons.
<a class="button is-default">Default</a> <a class="button is-primary">Primary</a> <a class="button is-secondary">Secondary</a> <a class="button is-success">Success</a> <a class="button is-danger">Danger</a> <a class="button is-warning">Warning</a> <a class="button is-inverse">Inverse</a> <a class="button is-link">Link</a>