In fulfilling #335 , we will have some basic, customizable progress bars built in to <model-viewer>. @yuinchien has described the default look and feel of these bars as including a linear gradient that is color-matched to the configured background-color of the element (or possibly the poster). See https://yuinchien.github.io/sandbox/progressbar/ for a reference of how the linear gradient should look and change over the course of the progress bar advancement.

We should implement this look and behavior as the default for our progress bars.