When is "DOM content loaded" event fired?

The "DOM Content Loaded" event is fired when the browser has fully parsed the main document and constructed the Document Object Model (DOM) tree. At this point, the HTML has been completely loaded, and any scripts that are designated to run at this event can execute, allowing developers to manipulate or access the DOM elements prior to loading additional resources like images, stylesheets, or frames.

The significance of this event lies in the fact that it signifies that the basic structure of the document is ready to be interacted with by scripts. This allows developers to perform actions like attaching event listeners or initiating dynamic content updates as soon as the essential HTML elements are available, improving the responsiveness of the web application.

The other options do not accurately describe this event. The server starting to send data pertains to the initial stage of the loading process and does not correlate with DOM readiness. Similarly, the event concerning the page being fully loaded includes resources beyond the DOM itself, such as images and styles, which does not occur until the "load" event. User interactions with the page occur after the DOM is fully loaded, whereas this event specifically relates to the completion of parsing the main document regardless of user actions.