video output to stream in mp4 format fail

Question

I'm trying to output mp4 file from the input stream

ffmpeg -f mp4 -i pipe:0 -an -c:v libx264 -crf 18 -s 400x400 -q:v 3 -f mp4 pipe:1

and I'm getting

[mp4 @ 0x55a646cca340] muxer does not support non seekable output Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument Error initializing output stream 0:0 -- Conversion failed!

When I convert to mpeg it works fine.

ffmpeg -f mp4 -i pipe:0 -an -c:v libx264 -crf 18 -s 400x400 -q:v 3 -f mpeg pipe:1

Is this mean that I cannot output to mp4? if so what is the best practice to convert video to be accepted from most browsers today?