How to Use macOS Mojave's New Dynamic Desktop Feature
У вашего броузера проблема в совместимости с HTML5
Apple in macOS Mojave introduced Dynamic Desktops, which are wallpapers that shift with the time of day, changing the lighting and look of the wallpaper with the progress of the sun across the sky.
Read more - https://www.macrumors.com/how-to/use-dynamic-desktop-macos-mojave/