run with "ruby main.rb GAME_ID" and build a simple webpage with all the not-obtained steam achievements for a game, ordered from the most popular to the less popular

main.rb

require 'ostruct'
require 'erb'

STEAM_PROFILE_ID = 'YOUR_STEAM_PROFILE_ID' # https://steamid.io/
STEAM_API_KEY = 'YOUR_API_KEY' # http://steamcommunity.com/dev/apikey (enter '127.0.0.1' if you're unsure about your domain name)

OUTPUT_HTML = 'steam_achievements.html'
INPUT_ERB = 'steam_achievements.html.erb'

module Steam
  require 'net/http'
  require 'json'

  STEAM_API_BASE_URI = 'api.steampowered.com'
  GET_GLOBAL_ACHIEVEMENT_PERCENTAGE_FOR_APP_ENDPOINT = '/ISteamUserStats/GetGlobalAchievementPercentagesForApp/v0002/'
  GET_PLAYER_ACHIEVEMENTS_ENDPOINT = '/ISteamUserStats/GetPlayerAchievements/v0001/'
  GET_SCHEMA_FOR_GAME_ENDPOINT = '/ISteamUserStats/GetSchemaForGame/v2/'

  class Client
    def initialize(api_key:)
      @api_key = api_key
    end

    def schema_for_game(game_id:)
      get("#{GET_SCHEMA_FOR_GAME_ENDPOINT}?appid=#{game_id}&key=#{@api_key}&format=json&l=en")
    end

    def all_achievements_for_game(game_id:)
      get("#{GET_GLOBAL_ACHIEVEMENT_PERCENTAGE_FOR_APP_ENDPOINT}?gameid=#{game_id}")
    end

    def player_achievements(game_id:, profile_id:)
      get("#{GET_PLAYER_ACHIEVEMENTS_ENDPOINT}?appid=#{game_id}&key=#{@api_key}&steamid=#{profile_id}&l=en")
    end

    private

    def get(endpoint)
      response = Net::HTTP.get_response(STEAM_API_BASE_URI, endpoint)
      if response.kind_of?(Net::HTTPSuccess)
        JSON.parse(response.body, object_class: OpenStruct)
      else
        response.error!
      end
    end
  end
end

game_id = ARGV[0]
profile_id = STEAM_PROFILE_ID
steam_client = Steam::Client.new(api_key: STEAM_API_KEY)

all_achievements = steam_client.all_achievements_for_game(game_id: game_id).achievementpercentages.achievements
own_achievements = steam_client.player_achievements(game_id: game_id, profile_id: profile_id).playerstats.achievements
achievements_icons = steam_client.schema_for_game(game_id: game_id).game.availableGameStats.achievements

@own_achievements_with_global_percentages_and_icons = own_achievements.map do |achievement|
  if achievement.achieved == 0
    achievement_icon = achievements_icons.find { |a| a.name == achievement.apiname }
    OpenStruct.new(
      id: achievement.apiname,
      name: achievement.name,
      description: achievement.description,
      percentage: all_achievements.find { |a| a.name == achievement.apiname }.percent,
      icon: achievement_icon.icon,
      incomplete_icon: achievement_icon.icongray
    )
  end
end.compact.sort_by(&:percentage).reverse!

template = File.read(INPUT_ERB)
result = ERB.new(template).result(binding)

File.open(OUTPUT_HTML, 'w+') { |f| f.write(result) }

steam_achievements.html.erb

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet">
<div class="container">
  <% @own_achievements_with_global_percentages_and_icons.each do |achievement| %>
      <div class="card col-md-4">
        <img class="card-img-top" src="<%= achievement.incomplete_icon %>" /><br />
        <div class="card-block">
          <h4 class="card-title"><%= achievement.name %><br /></h4>
          <p class="card-text">Description: <%= achievement.description %><br />
          <%= achievement.percentage.round(2) %> of all players</p>
        </div>
      </div>
  <% end %>
</div>