In this paper, we describe how cookie banners, as a consent mechanism in web applications, should be designed and implemented to be compliant with the ePrivacy Directive and the GDPR, defining 22 legal requirements. While some are provided by legal sources, others result from the domain expertise of computer scientists. We perform a technical assessment of whether technical (with computer science tools), manual (with a human operator) or user studies verification is needed. We show that it is not possible to assess legal compliance for the majority of requirements because of the current architecture of the web. With this approach, we aim to support policy makers assessing compliance in cookie banners, especially under the current revision of the EU ePrivacy framework.