QML

It arrived with Qt 4.7 in 2010, during the Nokia era, in response to how long it took to build touch interfaces in C++. An element is written as Rectangle { width: parent.width / 2 }, and when parent.width changes the width updates on its own. Property bindings are the whole point — the same reactive idea that later made React and Vue popular, but here from the start and aimed at native apps.

Logic is written in JavaScript for fast iteration and in C++ when it needs to be quick, with Qt's meta-object system bridging the two. QML drives KDE Plasma, car instrument clusters from several manufacturers, and Jolla's Sailfish. The acronym now officially stands for "Qt Modeling Language"; the original expansion turned out to be too narrow.