Assessment of Web User Interfaces is an important activity for engineering usable frontends and guiding design decisions that can be automated to reduce expertise demands, costs and time. However, existing metrics and models are too scattered and prototypical to find wider adoption in industry practice and require consolidation. Even the most advanced assessment platforms are limited in their computational performance, handling of inputs and results, and extensibility. By applying Web Engineering knowledge and principles, we devise a novel web-based platform that provides unified access to state-of-the-art assessments and addresses the identified shortcomings of existing solutions. We address the technical challenges of improving the assessment performance and supporting extensibility to facilitate the integration of new assessment models. Our experiments with 14 assessment models in two different scenarios show that implementing such a platform is feasible and that the assessment performance for multiple assessments can be improved by more than 55% through shared preprocessing and parallel computation.