Pixel / Tracking Pixel

tracking pixel · tracking tag · web beacon · 1x1 pixel · pixel tag · email open pixel

Definition

A tracking pixel is an invisible element embedded in a page or an email that reports an event — an open, a visit — to an analytics server as it loads; it is the historical predecessor of the modern event API.

What a tracking pixel is

A tracking pixel is a 1×1 element (a PNG or GIF) embedded in the HTML of a page or an email with a src attribute pointing at an analytics or advertising server. As the page loads, the browser requests that image and passes the event parameters through the URL.

<!-- A tracking pixel -->
<img src="https://tracker.example.com/pixel.gif?uid=123&event=view&page=pdp"
     width="1" height="1" style="display:none;" />

The server receives the GET request with its parameters and logs the event. The visitor sees nothing.

Pixels in advertising

Modern advertising pixels are in fact JavaScript libraries that set third-party cookies and send structured events:

PageView — a page visit
AddToCart — an item added to the basket
Purchase — a completed purchase with its value

The ad platform uses that data for two jobs: attributing conversions to campaigns and building lookalike audiences for targeting.

Pixel	Primary job	Data type
Meta Pixel	Attribution + lookalike	Behaviour + transactions
TikTok Pixel	Retargeting + lookalike	Behaviour + transactions
Google Tag	Conversions + remarketing	Transactions + events

Pixels in email: open tracking

In email campaigns the pixel answers a specific question: did the recipient open the message? The HTML email contains an <img> with a unique URL. When the recipient opens the message and the client loads images, the server records an open.

Limits: since iOS 15, Apple Mail loads images through its own proxy as the message arrives, regardless of what the recipient does. Open data from Apple devices has become unreliable: the same person can register three opens without ever reading the message.

Important: measure email engagement with clicks, which are a more reliable metric. The open rate is indicative, not exact.

Pixel or event API: how to choose

A pixel (client side) is simple to install but bounded by the browser: blockers, third-party cookie restrictions, no data at all when JavaScript is disabled.

An event API (server side) sends data straight from your server to the ad platform or analytics system, bypassing the browser. It is more reliable, unaffected by blockers and able to carry fuller transaction data.

Large retailers combine both: a client-side pixel for speed and a server-side API for reliability, with event deduplication between them.

Frequently asked questions

How does a pixel differ from a JavaScript snippet?

A pixel is passive: it sends a GET request on load with a limited set of parameters. A JavaScript snippet is active: it can read page context, manage cookies, send arbitrary events with a rich schema and work asynchronously. Modern ad pixels such as the Meta Pixel are in fact JavaScript snippets.

Why do browsers block pixels?

Most modern browsers block third-party cookies and tracking requests by default or through extensions. For email pixels, mail clients — Apple Mail from iOS 15 onwards — load images through their own proxy, which makes open data unreliable.

Why use a pixel in email when click tracking exists?

A pixel records that the message was opened — before any click. That allows segmentation by engagement: opened but did not click versus opened and clicked. Because of Apple Mail proxy caching, open data from iOS devices should be read with caution.

Do visitors have to be told about pixels?

Under GDPR and comparable regimes, yes. Tracking pixels on the site and in email must be described in the privacy policy. Advertising pixels require explicit consent in jurisdictions with restrictive regulation.