Bootstrap is Twitter's toolkit for creating rich and more consistent web interfaces across browsers and devices. The Bootstrap ecosystem offers, among other things, an iconic font, themes, a customization generator, themes for Wordpress / Drupal + / Joomla, templates for Fireworks / Photoshop, a button generator and a jQuery UI theme. How to get started? Check out this tutorial or this series or even this. Built With Bootstrap has lots of examples on how developers are putting Bootstrap to good use.
Tilt is a Firefox add-on that lets you visualize a web page's DOM (document object model) in 3D. In effect, you can see all the layers in a web page stacked upon each other with this. [more inside]
What the Heck is Shadow DOM? Browser developers realized that coding the appearance and behavior of HTML elements completely by hand is a) hard and b) silly. So they sort of cheated. They created a boundary between what you, the Web developer can reach and what’s considered implementation details, thus inaccessible to you. The browser however, can traipse across this boundary at will.