Web Experiment: Cross-subdomain experimentation

Description: Web experiments can now target multiple subdomains.

Why: For web experiments, customers want to be able to run experiments consistently across their websites (e.g. a.example.com and b.example.com). Web experiments now maintain a consistent identity (i.e. bucketing id, behavioral targeting events) across the same eTLD+1 (e.g. *.example.com), which allows end-users to be consistently assigned variants across subdomains.

Where: Web experiment

Available in EUDC: Yes

When: Enabled by default for web experiments created after 13th July 2026