Observe-Browsing-Topics header

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Non-standard: This feature is not standardized. We do not recommend using non-standard features in production, as they have limited browser support, and may change or be removed. However, they can be a suitable alternative in specific cases where no standard option exists.

Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

Warning: This feature is currently opposed by two browser vendors. See the Standards positions section for details of opposition.

The HTTP Observe-Browsing-Topics response header is used to mark topics of interest inferred from a calling site's URL (i.e., the site where the ad tech <iframe> is embedded) as observed in the response to a request generated by a feature that enables the Topics API. The browser will subsequently use those topics to calculate top topics for the current user for future epochs.

See Using the Topics API for more details.

Header type	Response header

Syntax

http

Observe-Browsing-Topics: ?1

Directives

?1: A character sequence declaring that topics of interest inferred from a calling site's URL (i.e., the site where the ad tech <iframe> is embedded) are marked as observed. The browser will subsequently use those topics to calculate topics of interest for a user for future epochs.