> They have perfectly great existing hardware decoder offloading APIs via the various OS' native APIs for videos
One vendor specific API, not "various OS' native APIs".
Firefox currently supports hardware video decoding with Intel's vendor specific VA-API only on Linux, which is not supported by NVIDIA. (A third-party VA-API to NVDEC translation layer for Linux does exist on GitHub, nvidia-vaapi-driver, but it's not yet reliable as the officially supported VDPAU or NVDEC, and is not included in official linux package repositories.)
Intel has VA-API, AMD has AMF, and NVIDIA has VDPAU which is being replaced by NVDEC/NVENC.
The idea behind Vulkan Video Extensions is to have a vendor independent and cross-platform video API.
One vendor specific API, not "various OS' native APIs".
Firefox currently supports hardware video decoding with Intel's vendor specific VA-API only on Linux, which is not supported by NVIDIA. (A third-party VA-API to NVDEC translation layer for Linux does exist on GitHub, nvidia-vaapi-driver, but it's not yet reliable as the officially supported VDPAU or NVDEC, and is not included in official linux package repositories.)
Intel has VA-API, AMD has AMF, and NVIDIA has VDPAU which is being replaced by NVDEC/NVENC.
The idea behind Vulkan Video Extensions is to have a vendor independent and cross-platform video API.