func Accepts[In, Out any](json func(in In) (Out, error), html func(in In) error) http.Handler {
  // call appropriate function depending on Accepts header
}

type Input struct {}
type Output struct {}

func indexJson(in *Input) (Output, error) {
  // render json
}

func indexHtml(in *Input) error {
  // render html
}

// Usage
router.Get("/", Accepts(indexJson, indexHtml))

type Accepts[In, Out any] struct {
  JSON func(in In) (Out, error)
  HTML func(in In) error
}

func (a *Accepts) ServeHTTP(w, r) {
}

// Usage
router.Get("/", Accepts[*Input, Output]{indexJson, indexHtml})

router.Get("/", Accepts{indexJson, indexHtml})