You know the request: "Can you make our website look modern?" The site in question is years old. It has jQuery plugins bound to class names, a checkout nobody wants to touch, inline styles toggled by scripts written by someone who left long ago, and a CMS or backend that generates the markup.

WordPress sites are an especially common case. Many run a theme plus a long list of plugins for forms, shops, bookings, memberships, SEO and page building. Each plugin brings its own markup, styles and scripts. Together, they often represent years of work and a large investment in setup, content and customization. Their owners want a fresh look, but above all they need every one of those plugins to keep working.

A rebuild means migrating all of that and retesting everything.…