Panorama
Not to be confused with panoramic images.
Panorama is a Valve-developed UI framework. It is heavily influenced by and closely resembles modern web authoring (HTML5/CSS/JS). It enables rapid development, high quality, and high performance interfaces as well as seamless integration with game content (3D models, particles, etc.)
Games
Panorama is available in all 
Source games after 
Counter-Strike: Global Offensive, and is in all 
Source 2 games.
Panorama Overview
See Dota 2 Workshop Tools/Panorama for a comprehensive guide.
A Panorama UI is made up out of elements called Panels. These Panels form the building blocks of the UI (labels, images, buttons, etc). The way the Panels are laid out on the UI is described in .XML layout files, and their appearance is described in .CSS style sheets.
To communicate with the game Panorama uses the JavaScript scripting language. Scripts also enable creation of more complex UI effects and behavior than is possible with CSS style sheets alone. Scripts can either be separate .JS files or added inline in the .XML layout files. A separate Event system also allows communication between the panels within the same UI.
