A tracking pixel is an image one pixel across, fetched from a server the moment a message is opened. That fetch is the whole legal matter: it happens on the reader's own device. This lists every remote resource an email carries, separates the ones that reach a server from the ones that do not, and marks the addresses that carry an identifier tying the open to a single recipient.
The source is parsed with DOMParser. The document that produces has no browsing context: it loads nothing and runs nothing, so inspecting an email here does not trigger the very pixels being looked for. Nothing is transmitted or stored. The legal notes reflect the CNIL recommendation of 14 April 2026 and its FAQ of 22 July 2026.