A computer-implemented system is provided that enables synthetic testing of web applications using monitoring agents instrumented in web content. A playback engine is configured to receive a test script for testing a given web page and initiates loading of the given web page by a web browser. A monitoring agent has a tag embedded into a document object model of the given web page and is a script defined in accordance with the scripting language. During playback of the test script by the playback agent, the monitoring agent determines a performance metric for the given web page. A wait agent has a tag embedded into the markup language of the given web page. During building of the document object model for the given web page, operates to inject the tag for the monitoring agent into the document object model for the given web page.