HTTPLeaks
All possible ways, a website can leak HTTP requests
...Its primary artifact is a single HTML file containing many combinations of elements and attributes that may trigger network activity. Researchers can use it to evaluate Content Security Policy enforcement, webmail sanitization, proxy rewriting, and privacy controls. The project highlights cases where opening content may unexpectedly disclose that it was viewed or reveal a user's direct network path. Keeping the cases together makes overlooked request-producing behaviors easier to compare. Contributors can add newly discovered leak sources as browser capabilities evolve. The repository is intended for authorized browser, email, and web security testing.