The Page-Load Lifecycle

When a browser opens a page it requests the HTML, parses it into the DOM, and runs scripts as it goes. GTM is added high in the page and loads asynchronously, then fires a sequence of built-in events as the page progresses.

The built-in events

• gtm.js, the container has loaded and can start firing.
• gtm.dom, the DOM is ready (the Page View / DOM Ready trigger).
• gtm.load, the window has fully loaded, images and all.

Timing matters because a tag that reads page content must fire after that content exists, and a tag that needs page context needs that context pushed before it runs. Choosing the right moment is half of getting a tag to work.