It is a lightweight (4kb) JavaScript library that can help web-developers, especially HTML/CSS coders, to easily write CSS or JavaScript code for any specific browser without any CSS hacks. It is indispensable solution for building cross-browser and cross-platform web-sites and web-applications.
How it works? Very simple. It generates a list of classes with the detailed information about browser, layout engine, OS, device and then attach it to <html> tag.
Unlike well-known Modernizr, the CSS/JS Browser Determiner is mostly focused not on the browser features but on the browser and the device itself. But it also determines the support of common CSS features.