Afaik nvidia's NVCUVID API only supports H264 encoding, which would require a JavaScript H264 decoder. So far I haven't been able to get the one JS H264 decoder I know of (Broadway) to work reliably and fast enough for large video sizes.

I wish the industry would just get their act together and support a common video codec in browsers along with a JavaScript API that can deal with decoding single frames. Currently native streaming support for the <video> element in browsers is extremely poor and proposed solutions like MPEG-DASH or HLS add around 10 seconds(!) of latency.