Html validation is a method of ensuring a good coding praxis for doing web development, it is also a way of ensuring the adherence to the chosen specification and furthering the cross compatibility of the code. In this study we investigate which are the most commonly occurring html validation errors, their potential effects and causes in a large variation of websites. We automatically fetch a large amount of web pages whose html code was then fed into a html validator for a static code review resulting in a comprehensive list of validation errors. The 20 most common validation errors are specified, general explanations to the reasons behind and solutions to these validation errors are given. Additionally we also discuss potential ramifications of the current state of html validation. We conclude that approximately 98.5% of all web pages we tested had some sort of validation error. We also conclude that the most commonly occurring type of errors are related to either missing or erroneous attributes or attribute values.