CSS animations can be applied to the <progress> element itself, but not to the anonymous child element that represents the internal colored bar. CSS animations are necessary for the internal bar element because animations are used to show that <progress> is in an indeterminate state. All of the different indeterminate animations currently used by <progress> are generated by the native theme code: there is no CSS-only fallback. This means that there is no indeterminate animation when there is no native theme support (Windows Classic or other platforms for which a native rendering was not implemented) or when the presence of author styling disables the native appearance. Coupled with the lack of CSS animation support for the internal bar element, this means that indeterminate animations are currently impossible for these cases.