It kind of makes sense on many BIOS/UEFI-less systems where e.g. Uboot is used. And it does contain things like kernel images, sometimes initRD files etc. (which may not be bootloader files but are still system boot files).

deleted by creator

Which does not mean that people like A.S. Tanenbaum should not be deserving recognition. With other words, your comment does not contribute to this thread.

For the uninitiated: his wife was murdered by him.

OP states to have the Pi already.

.webm is but a container, so it cannot be considered (in)efficient on its own, only when considering the video and audio “formats” (no expert on the terminology) within (probably VP8 or 9 for video).

Plan 9 is also monolithic, according to wikipedia. For BSD it depends.