WebAssembly: The target architecture, similar to x86 or ARM. Also a marketing term where "for WebAssembly" means deployed on the web targetting web browsers.
Emscripten: This is the SDK, which provides the C++ toolchain and runtime, as well as C++ wrappers for the HTML5 API.
HTML5: This is the platform target, similar to macOS or Windows. The QtGui platform plugin especially will make use of HTML5 features.
Current naming in Qt:
Project Name: Qt for WebAssembly
Jira platform/epic: webassembly
Platform plugin: html5/
Platform plugin class prefix: Qhtml5
Platform define Q_OS_HTML5
.pro files: html5
mkspec name: emscripten
Should we change any of these before it's too late?