Note that DOMContentLoaded will only fire if document.readyState is still in the loading state. By the time it is set to interactive (or later complete) the DOMContentLoaded event has already been fired.
see
Note that DOMContentLoaded will only fire if document.readyState is still in the loading state. By the time it is set to interactive (or later complete) the DOMContentLoaded event has already been fired.
see