I tried an Internet search ( hint! ) and came up with this among many hits:
There is a way to control these elements - such as media player controls - with what is called the "Shadow DOM" - which does not work in this case. The solutions I saw (as per the link above) "fixed" this issue by making the video~audio container's overflow invisible and then made the controls panel wider until the download button was outside the container bounds.
Also see this link about the download button in Chrome "situation".
This is a classic "kludge" & may not be a good solution in the long term.
width: calc(100% + 30px); /* Adjust as needed */
Edit: Looks like Mark beat me to it!