A system sends ad content to a publisher, where the ad content includes a first snippet of code. The system receives requests from browsers accessing a document at the publisher that includes the ad content, where the browsers send the requests based on execution of the first snippet of code. The system transmits, based on receipt of the requests, a second snippet of code to the browsers that instructs the browsers on a content, format and appearance of text to be inserted within the document when the browsers render the document. The system tracks each transmission of the second snippet of code to determine a number of times the text has been displayed at the browsers.