: The Image Embed Element - HTML - MDN Web Docs - Mozilla

specifies that you want the browser to send an Attribution-Reporting-Eligible header along with the image request.

On the server-side this is used to trigger sending an Attribution-Reporting-Register-Source or Attribution-Reporting-Register-Trigger header in the response, to register an image-based attribution source or attribution trigger, respectively. Which response header should be sent back depends on the value of the Attribution-Reporting-Eligible header that triggered the registration.

The corresponding source or trigger event is set off once the browser receives the response containing the image file.

Note: See the Attribution Reporting API for more details.

There are two versions of this attribute that you can set:

• Boolean, i.e. just the attributionsrc name. This specifies that you want the Attribution-Reporting-Eligible header sent to the same server as the src attribute points to. This is fine when you are handling the attribution source or trigger registration on the same server. When registering an attribution trigger this property is optional, and a boolean value will be used if it is omitted.
• Value containing one or more URLs, for example:

html<img src="image-file.png" alt="My image file description" attributionsrc="https://a.example/register-source https://b.example/register-source" />

This is useful in cases where the requested resource is not on a server you control, or you just want to handle registering the attribution source on a different server. In this case, you can specify one or more URLs as the value of attributionsrc. When the resource request occurs the Attribution-Reporting-Eligible header will be sent to the URL(s) specified in attributionSrc in addition to the resource origin. These URLs can then respond with a Attribution-Reporting-Register-Source or Attribution-Reporting-Register-Trigger header as appropriate to complete registration.

Note: Specifying multiple URLs means that multiple attribution sources can be registered on the same feature. You might for example have different campaigns that you are trying to measure the success of, which involve generating different reports on different data.