That's why I use portfolio assessment. It enables students to create a narrative of their learning, and allows me insight into the process of that development. Evaluation of web projects can be difficult. Setting up clear guidelines for students will help, but it won't be enough. I make it clear to my students that I have a number of expectations regarding their web designs, but good design is not a replacement for content.
Content is what counts.
I look back now and blame myself for their lack of design, but at the same time I must remember these were the days before color backgrounds and handy HTML editors that do it all for you. As more transparent software develops, I'm sure my teaching will shift, but the expectations for good work will not change. My students contribute to the evaluative process in that they evaluate each other's work. I have a handout with space for comments that students produce for each project, including their own. But the best way I have found to evaluate web work is to have students evaluate themselves.