Set this up in a crontab to check if a webpage changed since last check. It will email you the diff when it happens.

watch-for-webpage-change-cron.sh

#!/bin/sh

checkIfPageChanged() {
  URL="$1"
  EMAIL_ADDRESS="$2"
  CSS_SELECTOR="$3"
  SILENT="$4"
  tmpfile1=/tmp/$(echo $URL| sha1sum | cut -d" " -f1)
  tmpfile2=$tmpfile1-1
  curl -L --silent "$URL" -A "Maxme/1.0 (Watching for changes; https://gist.github.com/maxme/c31ffbb3cb21f3baa6e9c9e61bbe0a72)" \
    | hxclean 2> /dev/null \
    | hxselect "$CSS_SELECTOR" > $tmpfile1 2> /dev/null
  diff $tmpfile2 $tmpfile1 > /dev/null 2> /dev/null
  if [ $? -eq 1 ]; then
    if [ x"$SILENT" = x ]; then
      echo "$URL was modified since last check"
    fi
    diff -u $tmpfile2 $tmpfile1 | mail -s "[modified web page] $URL" "$EMAIL_ADDRESS"
  fi
  mv $tmpfile1 $tmpfile2
}

if [ x"$3" = x ]; then
  echo "Usage: $0 URL EMAIL_ADDRESS SLEEP_TIME_SEC CSS_SELECTOR [silent]"
  echo "Example: $0 https://github.com/maxme/bitcoin-arbitrage/pulls pr@bia.is .issues-listing"
  exit 1
fi

checkIfPageChanged "$1" "$2" "$3" "$4"