Lea Verou proposes a "smart" dual-state dark mode toggle for websites, advocating for simplicity over the typical three options (dark, light, system default). The challenge lies in accommodating the system default, which can vary. Many current toggles pass this complexity to the user.

Verou's proposed toggle offers only two visible options: light and dark. Clicking the toggle switches to the opposite theme. If the chosen theme differs from the system's current setting, it locks into that theme. If it matches the system, it reverts to following the system's default behavior. This approach aims to abstract away the underlying complexity.

The author acknowledges that this "smart" toggle might have rare, perhaps imaginary, use cases it doesn't fully support. However, they agree with Verou's stance against implementation-driven UI, emphasizing the importance of designing for user goals. The "system default" option is often unclear and adds cognitive load, unlike the straightforward light and dark choices.

This seemingly small UX improvement is presented as a valuable exercise in managing complexity. Just as modern computers have simplified power buttons, Verou's smart toggle offers a more intuitive user experience by hiding technical intricacies, ultimately leading to better design.