diff --git a/lib/rack/handler.rb b/lib/rack/handler.rb
index df17b238d..22d547453 100644
--- a/lib/rack/handler.rb
+++ b/lib/rack/handler.rb
@@ -31,7 +31,7 @@ def self.get(server)
     # Select first available Rack handler given an `Array` of server names.
     # Raises `LoadError` if no handler was found.
     #
-    #   > pick ['thin', 'webrick']
+    #   > pick ['puma', 'webrick']
     #   => Rack::Handler::WEBrick
     def self.pick(server_names)
       server_names = Array(server_names)
@@ -45,17 +45,14 @@ def self.pick(server_names)
       raise LoadError, "Couldn't find handler for: #{server_names.join(', ')}."
     end
 
-    SERVER_NAMES = %w(puma thin falcon webrick).freeze
+    RACK_HANDLER = 'RACK_HANDLER'
+
+    SERVER_NAMES = %w(puma falcon webrick).freeze
     private_constant :SERVER_NAMES
 
     def self.default
-      # Guess.
-      if ENV.include?("PHP_FCGI_CHILDREN")
-        Rack::Handler::FastCGI
-      elsif ENV.include?(REQUEST_METHOD)
-        Rack::Handler::CGI
-      elsif ENV.include?("RACK_HANDLER")
-        self.get(ENV["RACK_HANDLER"])
+      if rack_handler = ENV[RACK_HANDLER]
+        self.get(rack_handler)
       else
         pick SERVER_NAMES
       end
@@ -87,18 +84,7 @@ def self.register(server, klass)
       @handlers[server.to_s] = klass.to_s
     end
 
-    autoload :CGI, "rack/handler/cgi"
-    autoload :FastCGI, "rack/handler/fastcgi"
     autoload :WEBrick, "rack/handler/webrick"
-    autoload :LSWS, "rack/handler/lsws"
-    autoload :SCGI, "rack/handler/scgi"
-    autoload :Thin, "rack/handler/thin"
-
-    register 'cgi', 'Rack::Handler::CGI'
-    register 'fastcgi', 'Rack::Handler::FastCGI'
     register 'webrick', 'Rack::Handler::WEBrick'
-    register 'lsws', 'Rack::Handler::LSWS'
-    register 'scgi', 'Rack::Handler::SCGI'
-    register 'thin', 'Rack::Handler::Thin'
   end
 end
diff --git a/lib/rack/handler/cgi.rb b/lib/rack/handler/cgi.rb
index 1c11ab360..48da2e2fc 100644
--- a/lib/rack/handler/cgi.rb
+++ b/lib/rack/handler/cgi.rb
@@ -55,5 +55,7 @@ def self.send_body(body)
         }
       end
     end
+    
+    register 'cgi', 'Rack::Handler::CGI'
   end
 end
diff --git a/lib/rack/handler/fastcgi.rb b/lib/rack/handler/fastcgi.rb
deleted file mode 100644
index 1df123e02..000000000
--- a/lib/rack/handler/fastcgi.rb
+++ /dev/null
@@ -1,100 +0,0 @@
-# frozen_string_literal: true
-
-require 'fcgi'
-require 'socket'
-
-if defined? FCGI::Stream
-  class FCGI::Stream
-    alias _rack_read_without_buffer read
-
-    def read(n, buffer = nil)
-      buf = _rack_read_without_buffer n
-      buffer.replace(buf.to_s)  if buffer
-      buf
-    end
-  end
-end
-
-module Rack
-  module Handler
-    class FastCGI
-      def self.run(app, **options)
-        if options[:File]
-          STDIN.reopen(UNIXServer.new(options[:File]))
-        elsif options[:Port]
-          STDIN.reopen(TCPServer.new(options[:Host], options[:Port]))
-        end
-        FCGI.each { |request|
-          serve request, app
-        }
-      end
-
-      def self.valid_options
-        environment  = ENV['RACK_ENV'] || 'development'
-        default_host = environment == 'development' ? 'localhost' : '0.0.0.0'
-
-        {
-          "Host=HOST" => "Hostname to listen on (default: #{default_host})",
-          "Port=PORT" => "Port to listen on (default: 8080)",
-          "File=PATH" => "Creates a Domain socket at PATH instead of a TCP socket. Ignores Host and Port if set.",
-        }
-      end
-
-      def self.serve(request, app)
-        env = request.env
-        env.delete "HTTP_CONTENT_LENGTH"
-
-        env[SCRIPT_NAME] = ""  if env[SCRIPT_NAME] == "/"
-
-        rack_input = RewindableInput.new(request.in)
-
-        env.update(
-          RACK_VERSION      => Rack::VERSION,
-          RACK_INPUT        => rack_input,
-          RACK_ERRORS       => request.err,
-          RACK_MULTITHREAD  => false,
-          RACK_MULTIPROCESS => true,
-          RACK_RUNONCE      => false,
-          RACK_URL_SCHEME   => ["yes", "on", "1"].include?(env[HTTPS]) ? "https" : "http"
-        )
-
-        env[QUERY_STRING] ||= ""
-        env[HTTP_VERSION] ||= env[SERVER_PROTOCOL]
-        env[REQUEST_PATH] ||= "/"
-        env.delete "CONTENT_TYPE"  if env["CONTENT_TYPE"] == ""
-        env.delete "CONTENT_LENGTH"  if env["CONTENT_LENGTH"] == ""
-
-        begin
-          status, headers, body = app.call(env)
-          begin
-            send_headers request.out, status, headers
-            send_body request.out, body
-          ensure
-            body.close  if body.respond_to? :close
-          end
-        ensure
-          rack_input.close
-          request.finish
-        end
-      end
-
-      def self.send_headers(out, status, headers)
-        out.print "Status: #{status}\r\n"
-        headers.each { |k, vs|
-          vs.split("\n").each { |v|
-            out.print "#{k}: #{v}\r\n"
-          }
-        }
-        out.print "\r\n"
-        out.flush
-      end
-
-      def self.send_body(out, body)
-        body.each { |part|
-          out.print part
-          out.flush
-        }
-      end
-    end
-  end
-end
diff --git a/lib/rack/handler/lsws.rb b/lib/rack/handler/lsws.rb
deleted file mode 100644
index f12090bd6..000000000
--- a/lib/rack/handler/lsws.rb
+++ /dev/null
@@ -1,61 +0,0 @@
-# frozen_string_literal: true
-
-require 'lsapi'
-
-module Rack
-  module Handler
-    class LSWS
-      def self.run(app, **options)
-        while LSAPI.accept != nil
-          serve app
-        end
-      end
-      def self.serve(app)
-        env = ENV.to_hash
-        env.delete "HTTP_CONTENT_LENGTH"
-        env[SCRIPT_NAME] = "" if env[SCRIPT_NAME] == "/"
-
-        rack_input = RewindableInput.new($stdin.read.to_s)
-
-        env.update(
-          RACK_VERSION      => Rack::VERSION,
-          RACK_INPUT        => rack_input,
-          RACK_ERRORS       => $stderr,
-          RACK_MULTITHREAD  => false,
-          RACK_MULTIPROCESS => true,
-          RACK_RUNONCE      => false,
-          RACK_URL_SCHEME   => ["yes", "on", "1"].include?(ENV[HTTPS]) ? "https" : "http"
-        )
-
-        env[QUERY_STRING] ||= ""
-        env[HTTP_VERSION] ||= env[SERVER_PROTOCOL]
-        env[REQUEST_PATH] ||= "/"
-        status, headers, body = app.call(env)
-        begin
-          send_headers status, headers
-          send_body body
-        ensure
-          body.close if body.respond_to? :close
-        end
-      ensure
-        rack_input.close
-      end
-      def self.send_headers(status, headers)
-        print "Status: #{status}\r\n"
-        headers.each { |k, vs|
-          vs.split("\n").each { |v|
-            print "#{k}: #{v}\r\n"
-          }
-        }
-        print "\r\n"
-        STDOUT.flush
-      end
-      def self.send_body(body)
-        body.each { |part|
-          print part
-          STDOUT.flush
-        }
-      end
-    end
-  end
-end
diff --git a/lib/rack/handler/scgi.rb b/lib/rack/handler/scgi.rb
deleted file mode 100644
index e3b8d3c6f..000000000
--- a/lib/rack/handler/scgi.rb
+++ /dev/null
@@ -1,71 +0,0 @@
-# frozen_string_literal: true
-
-require 'scgi'
-require 'stringio'
-
-module Rack
-  module Handler
-    class SCGI < ::SCGI::Processor
-      attr_accessor :app
-
-      def self.run(app, **options)
-        options[:Socket] = UNIXServer.new(options[:File]) if options[:File]
-        new(options.merge(app: app,
-                          host: options[:Host],
-                          port: options[:Port],
-                          socket: options[:Socket])).listen
-      end
-
-      def self.valid_options
-        environment  = ENV['RACK_ENV'] || 'development'
-        default_host = environment == 'development' ? 'localhost' : '0.0.0.0'
-
-        {
-          "Host=HOST" => "Hostname to listen on (default: #{default_host})",
-          "Port=PORT" => "Port to listen on (default: 8080)",
-        }
-      end
-
-      def initialize(settings = {})
-        @app = settings[:app]
-        super(settings)
-      end
-
-      def process_request(request, input_body, socket)
-        env = Hash[request]
-        env.delete "HTTP_CONTENT_TYPE"
-        env.delete "HTTP_CONTENT_LENGTH"
-        env[REQUEST_PATH], env[QUERY_STRING] = env["REQUEST_URI"].split('?', 2)
-        env[HTTP_VERSION] ||= env[SERVER_PROTOCOL]
-        env[PATH_INFO] = env[REQUEST_PATH]
-        env[QUERY_STRING] ||= ""
-        env[SCRIPT_NAME] = ""
-
-        rack_input = StringIO.new(input_body)
-        rack_input.set_encoding(Encoding::BINARY)
-
-        env.update(
-          RACK_VERSION      => Rack::VERSION,
-          RACK_INPUT        => rack_input,
-          RACK_ERRORS       => $stderr,
-          RACK_MULTITHREAD  => true,
-          RACK_MULTIPROCESS => true,
-          RACK_RUNONCE      => false,
-          RACK_URL_SCHEME   => ["yes", "on", "1"].include?(env[HTTPS]) ? "https" : "http"
-        )
-
-        status, headers, body = app.call(env)
-        begin
-          socket.write("Status: #{status}\r\n")
-          headers.each do |k, vs|
-            vs.split("\n").each { |v| socket.write("#{k}: #{v}\r\n")}
-          end
-          socket.write("\r\n")
-          body.each {|s| socket.write(s)}
-        ensure
-          body.close if body.respond_to? :close
-        end
-      end
-    end
-  end
-end
diff --git a/lib/rack/handler/thin.rb b/lib/rack/handler/thin.rb
deleted file mode 100644
index 393a6e986..000000000
--- a/lib/rack/handler/thin.rb
+++ /dev/null
@@ -1,36 +0,0 @@
-# frozen_string_literal: true
-
-require "thin"
-require "thin/server"
-require "thin/logging"
-require "thin/backends/tcp_server"
-
-module Rack
-  module Handler
-    class Thin
-      def self.run(app, **options)
-        environment  = ENV['RACK_ENV'] || 'development'
-        default_host = environment == 'development' ? 'localhost' : '0.0.0.0'
-
-        host = options.delete(:Host) || default_host
-        port = options.delete(:Port) || 8080
-        args = [host, port, app, options]
-        # Thin versions below 0.8.0 do not support additional options
-        args.pop if ::Thin::VERSION::MAJOR < 1 && ::Thin::VERSION::MINOR < 8
-        server = ::Thin::Server.new(*args)
-        yield server if block_given?
-        server.start
-      end
-
-      def self.valid_options
-        environment  = ENV['RACK_ENV'] || 'development'
-        default_host = environment == 'development' ? 'localhost' : '0.0.0.0'
-
-        {
-          "Host=HOST" => "Hostname to listen on (default: #{default_host})",
-          "Port=PORT" => "Port to listen on (default: 8080)",
-        }
-      end
-    end
-  end
-end
diff --git a/lib/rack/handler/webrick.rb b/lib/rack/handler/webrick.rb
index d2f389758..1e88c1956 100644
--- a/lib/rack/handler/webrick.rb
+++ b/lib/rack/handler/webrick.rb
@@ -37,7 +37,7 @@ def self.run(app, **options)
 
         @server = ::WEBrick::HTTPServer.new(options)
         @server.mount "/", Rack::Handler::WEBrick, app
-        yield @server  if block_given?
+        yield @server if block_given?
         @server.start
       end
 
diff --git a/test/spec_server.rb b/test/spec_server.rb
index 20992a0f9..a6968cfb9 100644
--- a/test/spec_server.rb
+++ b/test/spec_server.rb
@@ -8,6 +8,8 @@
 require 'net/http'
 require 'net/https'
 
+require 'rack/handler/cgi'
+
 module Minitest::Spec::DSL
   alias :should :it
 end
@@ -75,17 +77,6 @@ def with_stderr
     server.middleware['deployment'].flatten.must_include Rack::TempfileReaper
   end
 
-  it "support CGI" do
-    begin
-      o, ENV["REQUEST_METHOD"] = ENV["REQUEST_METHOD"], 'foo'
-      server = Rack::Server.new(app: 'foo')
-      server.server.name =~ /CGI/
-      Rack::Server.logging_middleware.call(server).must_be_nil
-    ensure
-      ENV['REQUEST_METHOD'] = o
-    end
-  end
-
   it "be quiet if said so" do
     server = Rack::Server.new(app: "FOO", quiet: true)
     Rack::Server.logging_middleware.call(server).must_be_nil