<.progress_bar_base>

The bar itself. `animated` decides who moves it. Unanimated, the server writes the fill's transform inline, which is right for a value that changes when a person does something. Animated, the browser writes it: the server only states the target, and the element eases towards it. That distinction matters for a bar driven by a stream of updates. Patching an inline transform restarts the transition from wherever it had got to, dozens of times a second, which reads as blinking and stuttering rather than motion. Letting the element own the transform means one continuous animation that simply retargets, however often or unevenly the updates arrive.