This document introduces the Archival Acid Test, which evaluates how well web archiving tools archive modern webpages that use advanced HTML, JavaScript, and other web technologies. The test is divided into basic tests, JavaScript tests, and advanced features tests to assess different areas. Results show that archiving tools perform well on basic tests but struggle with dynamic content, asynchronous JavaScript, iframes, and other complex features. The goal of the Archival Acid Test is to create a standardized, publicly available way to evaluate how completely archiving tools archive modern webpages and identify areas for improvement.