https://javascript.info/onload-ondomcontentloaded

DOMContentLoaded and scripts
When the browser processes an HTML-document and comes across a <script> tag, it needs to execute before continuing building the DOM. That’s a precaution, as scripts may want to modify DOM, and even document.write into it, so DOMContentLoaded has to wait.

Scripts that don’t block DOMContentLoaded
There are two exceptions from this rule:

Scripts with the async attribute, that we’ll cover a bit later, don’t block DOMContentLoaded.
Scripts that are generated dynamically with document.createElement('script') and then added to the webpage also don’t block this event.