And remember Quibi [1]? Short-form video in vertical format specifically for mobile devices? They didn't have every aspect nailed, but they were definitely trailblazers on that front.
Quibi launched in April 2020. TikTok by this point would have 2 billion downloads [1]. It's difficult to assess they were trailblazers here. I might even say a component of their failure is free mobile video was widely accessible by this point.