1. A method comprising: generating a layout definition based at least in part on a definition input, wherein the layout definition is a graphical data tree and the definition input is a template language; generating a behavior definition based at least in part on a behavior input, the behavior definition to describe how an element of one or more graphical user interfaces (“GUIs”) generated based on the layout definition behave; defining a style sheet based at least in part on a style input, the style sheet to describe a visual property of the element; and determining graphical data to generate the one or more GUIs based at least in part on the layout definition, the behavior definition and the style sheet.