Cross Browser

Simply-Buttons is a technique for creating nice looking buttons. You can easily modify the buttons look and feel by creating your own stylesheet.   Simply-Buttons - Features: Buttons size to fit text. There are 3 states: Inactive, Active, and Hover. They look and behave the same way in every browser and operating system. The do not need the javascript to work, they work with just the css  

Modernizr is a JavaScript library that detects HTML5 and CSS3 features in the user’s browser. This library makes it easy for you to write conditional JavaScript and CSS to handle each situation, whether a browser supports a feature or not. It’s perfect for doing progressive enhancement easily.  

Conditional-CSS allows you to write maintainable CSS with conditional logic to target specific CSS statements at both individual browsers and groups of browsers.     Conditional-CSS - Features: Target CSS to any web-browser Streamline maintenance of your CSS files Optimise your CSS Work around those annoying little CSS bugs Automatic expansion and inclusion of @import statements Free and open source!   Conditional-CSS is available in three different formats. Which format you will wis

HTML5 Placeholder is a jQuery plugin that enables HTML5 placeholder behavior for browsers that do not support it. This placeholder plugin works in all A-grade browsers, including IE6.