How can a web designer sell this to existing clients?

Start with the customer behaviour question: do your customers use AI before they buy? If yes, the business needs to be understandable to those AI systems. The Digital Twin is the structured answer to that problem. For web designers, the value is being able to add an AI-readable layer beside the client website without rebuilding the whole business from scratch. In other words, the goal is to give the client a controlled first AI layer instead of leaving the whole topic vague and undefined.