My attempt on writing script for ffmpeg on WEBM.

v9-ffmpeg.sh

#!/bin/bash

# Usage:
#   v9-ffmpeg.sh 720 input.mp4
#   v9-ffmpeg.sh 720 input.mp4 output.webm
#
# Best usage for scaling down video OR just simply reencode video into WEBM.

# Change me.
OUTPUT_PREFIX=""
# Set to 0 as it is.
OUTPUT_BASECRF="0"

__get_info() {
  local input="$1"
  local entry="$2"

  ffprobe -v error -select_streams v:0 -of default=nw=1:nk=1 \
    -show_entries stream="$entry" "$input"
}

__encode() {
  local target="$1"
  local input="$2"
  local output="$3"
  local w=$(__get_info "$input" "width")
  local h=$(__get_info "$input" "height")

  # https://developers.google.com/media/vp9/settings/vod/#recommended_settings
  local filter
  local scale
  local bitrate
  local minrate
  local maxrate
  local qcrf

  local tcolumn1=1
  local tcolumn2=1
  local threads=4

  case $target in
    1080)
      # For modern mobile device storing
      scale="1080"
      qcrf="19"
      tcolumn1=2
      tcolumn2=3
      threads=8
    ;;

    720)
      # For reasonable online sharing
      scale="720"
      qcrf="20"
      tcolumn1=2
      tcolumn2=2
      threads=8
    ;;

    480)
      # 10+ mins long or VGA
      scale="480"
      qcrf="24"
    ;;

    360)
      # Half of 720p
      scale="360"
      qcrf="28"
    ;;

    240)
      # Very 2000-ish (OR MEME QUALITY; DO NOT REMOVE)
      scale="240"
      qcrf="30"
    ;;

    *)
      echo "Preset doesn't exist."
      exit 1
    ;;
  esac

  # Output follows input with different file extension, relative to CWD
  if [ -z "$output" ]; then
    output=$(basename "${input%.*}.webm")
  fi

  # Portrait orientation?
  if [[ $h -gt $w ]]; then
    filter="scale=${scale}:-2"
  else
    filter="scale=-2:${scale}"
  fi

  # Recalculate crf base: 18 to 23 = lossless
  qcrf=$(( $OUTPUT_BASECRF + $qcrf ))
  # 35 is the maximum recommended
  qcrf=$(( $qcrf < 35 ? $qcrf : 35 ))
  # lesser than 18 is placebo
  qcrf=$(( $qcrf > 18 ? $qcrf : 18 ))

  # https://developers.google.com/media/vp9/settings/vod/#ffmpeg_command_lines
  ffmpeg -i "$input" -vf "$filter" \
    -tile-columns "$tcolumn1" -g 240 -threads "$threads" -quality good -crf "$qcrf" \
    -c:v libvpx-vp9 -c:a libopus -b:a 256k -pass 1 -speed 4 "${OUTPUT_PREFIX}${output}" && \
  ffmpeg -i "$input" -vf "$filter" \
    -tile-columns "$tcolumn2" -g 240 -threads "$threads" -quality good -crf "$qcrf" \
    -c:v libvpx-vp9 -c:a libopus -b:a 256k -pass 2 -speed 4 -y "${OUTPUT_PREFIX}${output}"
}

# Pass the args into function. This script only takes one file.
__encode "$@"

Omitted bitrate, minbitrate, and maxbitrate. Previous version can be viewed here: https://gist.github.com/gphg/1dd083b89bddd17548eb721d6475433c/474a465094e8102e6cea1e03d46ac9f3d12294b3

It is now tied to CRF clamp between 18 and 35.