Too Long; Didn't Read
About a decade ago or so, it was fashionable to model a GUI in a cross-platform flavor of XML (like XUL) and then have a platform-specific rendering engine process that XML-file to display a layout. It is no longer fashionable to do this, however. Today, in 2017, the trend is to build your apps on platforms that allow you to write everything in HTML, CSS and/or JavaScript, no matter what environment your code is supposed to run in.