Uniform Resource Locators(URLs) allow to quickly and precisely navigate today’s web. Similar to the specifications of other web standards, such as HTML, the WHATWG maintains the URL specification as a living standard. However, because different applications use URLs for a multitude of purposes, there exists a variety of implementations of URL parsers, most of which claim to follow the URL standard. This thesis uses grammar-based fuzzing together with a grammar of the current URL standard to examine how close the relationship between URL parsers and the standard is. In detail, this consists of testing the URL parsers included in the browsers Firefox and Chromium, as well as a selection of stand-alone URL parsers with inputs generated by executing a grammar-based fuzzer on the URL grammar. Additionally, this thesis evaluates the errors encountered during test execution as well as the code coverage achieved in the selected URL parsers.