Rapid prototyping front end libraries provide skeleton code for CSS, HTML, and JavaScript that developers can use for common UI elements in a project. Two popular options are Bootstrap and Foundation. Both provide a grid system, styles for forms, buttons, tables and other common patterns. They also include modal windows, image grids, drop down menus and other pre-built components. The main differences are that Bootstrap has a more polished, product-like appearance with transitions while Foundation provides a more raw foundation for customization and extension.