This paper introduces Senser, a system for validating retrieved web content. Senser does not rely on a PKI and operates even when SSL/TLS is not supported by the web server. Senser operates as a network of proxies located at different vantage points on the Internet. Clients query a random subset of Senser proxies for compact descriptions of a desired web page, and apply consensus and matching algorithms to the returned results to locally render a "majority" web page. To ensure diverse selections of proxies (and consequently decrease an adversary's ability to manipulate a majority of the proxies' requests), Senser leverages Internet mapping systems that accurately predict AS-level paths between available proxies and the desired web page. We demonstrate using a deployment of Senser on Amazon EC2 that Senser detects and mitigates attempts by adversaries to manipulate web content --- even when controlling large collections of autonomous systems --- while maintaining reasonable performance overheads.