Get this warning when running app.

Rules for selecting event targets in HTML5 API are changing: instead of using document.getElementById() that only can refer to elements by their DOM ID, new event target selection mechanism uses the more flexible function document.querySelector() that can look up element names, classes, and complex CSS selectors. Build with -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=1 to change to the new lookup rules. See https://github.com/emscripten-core/emscripten/pull/7977 for more details.