Using fluent-ffmpeg to quickly create an MP4->WebM transcode with VP8 video. Adds the psnr flag so that we can get the transcode's peak signal-to-noise ratio. Any result greater than 35dB would be acceptable for most streaming use cases.

FFmpeg PSNR

[fluent_ffmpeg] node ffmpeg.js                                                                                                                             
ffmpeg version N-75369-g5ba811b Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 5.2.0 (GCC)
  configuration: --prefix=/usr --shlibdir=/usr/lib --pkg-config-flags=--static --disable-static --enable-gpl --enable-gnutls --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-nonfree --enable-shared --enable-x11grab
  libavutil      55.  2.100 / 55.  2.100
  libavcodec     57.  2.100 / 57.  2.100
  libavformat    57.  2.100 / 57.  2.100
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6.  4.100 /  6.  4.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.100 /  2.  0.100
  libpostproc    54.  0.100 / 54.  0.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'http://grathwohl.me/futbol.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf55.33.100
  Duration: 00:00:11.52, start: 0.021333, bitrate: 1586 kb/s
    Stream #0:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 854x480 [SAR 1280:1281 DAR 16:9], 1491 kb/s, 23.98 fps, 23.98 tbr, 24k tbn, 47.95 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(por): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 92 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
[libvpx @ 0x27968e0] v1.4.0
[libopus @ 0x2c99320] No bit rate set. Defaulting to 96000 bps.
Output #0, webm, to 'futbol.webm':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf57.2.100
    Stream #0:0(und): Video: vp8 (libvpx), yuv420p, 854x480 [SAR 1280:1281 DAR 16:9], q=-1--1, 1000 kb/s, 23.98 fps, 1k tbn, 23.98 tbc (default)
    Metadata:
      handler_name    : VideoHandler
      encoder         : Lavc57.2.100 libvpx
    Stream #0:1(por): Audio: opus (libopus), 48000 Hz, stereo, flt, 96 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
      encoder         : Lavc57.2.100 libopus
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> vp8 (libvpx))
  Stream #0:1 -> #0:1 (aac (native) -> opus (libopus))
Press [q] to stop, [?] for help
frame=  276 fps= 33 q=0.0 LPSNR=Y:42.34 U:45.34 V:45.35 *:43.13 size=    1614kB time=00:00:11.55 bitrate=1144.5kbits/s    
video:1462kB audio:146kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.404469%

Processing finished.
This WebM transcode scored a PSNR of: 
43.13dB

psnr_ffmpeg.js

var ffmpeg = require('fluent-ffmpeg');
var vp8_webm = ffmpeg('http://grathwohl.me/futbol.mp4').videoCodec('libvpx') //libvpx-vp9 could be used too
        .videoBitrate(1000, true) //Outputting a constrained 1Mbit VP8 video stream
        .outputOptions(
                '-minrate', '1000',
                '-maxrate', '1000',
                '-threads', '3', //Use number of real cores available on the computer - 1
                '-flags', '+global_header', //WebM won't love if you if you don't give it some headers
                '-psnr') //Show PSNR measurements in output. Anything above 40dB indicates excellent fidelity
          .on('progress', function(progress) {
              console.log('Processing: ' + progress.percent + '% done');
        })
        .on('error', function(err) {
              console.log('An error occurred: ' + err.message);
                })
        .on('end', function(err, stdout, stderr) {
              console.log(stdout);
              console.log('Processing finished.');
              var regex = /LPSNR=Y:([0-9\.]+) U:([0-9\.]+) V:([0-9\.]+) \*:([0-9\.]+)/
              var psnr = stdout.match(regex);
              console.log('This WebM transcode scored a PSNR of: ');
              console.log(psnr[4] + 'dB');
                })
        .save('futbol.webm');