diff --git a/app/controllers/admin/settings_controller.rb b/app/controllers/admin/settings_controller.rb
index a763597f20..dc1c79b7fd 100644
--- a/app/controllers/admin/settings_controller.rb
+++ b/app/controllers/admin/settings_controller.rb
@@ -2,84 +2,29 @@
 
 module Admin
   class SettingsController < BaseController
-    ADMIN_SETTINGS = %w(
-      site_contact_username
-      site_contact_email
-      site_title
-      site_short_description
-      site_description
-      site_extended_description
-      site_terms
-      registrations_mode
-      closed_registrations_message
-      open_deletion
-      timeline_preview
-      show_staff_badge
-      bootstrap_timeline_accounts
-      theme
-      thumbnail
-      hero
-      mascot
-      min_invite_role
-      activity_api_enabled
-      peers_api_enabled
-      show_known_fediverse_at_about_page
-      preview_sensitive_media
-      custom_css
-      profile_directory
-    ).freeze
-
-    BOOLEAN_SETTINGS = %w(
-      open_deletion
-      timeline_preview
-      show_staff_badge
-      activity_api_enabled
-      peers_api_enabled
-      show_known_fediverse_at_about_page
-      preview_sensitive_media
-      profile_directory
-    ).freeze
-
-    UPLOAD_SETTINGS = %w(
-      thumbnail
-      hero
-      mascot
-    ).freeze
-
     def edit
       authorize :settings, :show?
+
       @admin_settings = Form::AdminSettings.new
     end
 
     def update
       authorize :settings, :update?
 
-      settings_params.each do |key, value|
-        if UPLOAD_SETTINGS.include?(key)
-          upload = SiteUpload.where(var: key).first_or_initialize(var: key)
-          upload.update(file: value)
-        else
-          setting = Setting.where(var: key).first_or_initialize(var: key)
-          setting.update(value: value_for_update(key, value))
-        end
-      end
+      @admin_settings = Form::AdminSettings.new(settings_params)
 
-      flash[:notice] = I18n.t('generic.changes_saved_msg')
-      redirect_to edit_admin_settings_path
+      if @admin_settings.save
+        flash[:notice] = I18n.t('generic.changes_saved_msg')
+        redirect_to edit_admin_settings_path
+      else
+        render :edit
+      end
     end
 
     private
 
     def settings_params
-      params.require(:form_admin_settings).permit(ADMIN_SETTINGS)
-    end
-
-    def value_for_update(key, value)
-      if BOOLEAN_SETTINGS.include?(key)
-        value == '1'
-      else
-        value
-      end
+      params.require(:form_admin_settings).permit(*Form::AdminSettings::KEYS)
     end
   end
 end
diff --git a/app/models/form/admin_settings.rb b/app/models/form/admin_settings.rb
index a21394a52a..2d3aa726d6 100644
--- a/app/models/form/admin_settings.rb
+++ b/app/models/form/admin_settings.rb
@@ -3,49 +3,90 @@
 class Form::AdminSettings
   include ActiveModel::Model
 
-  delegate(
-    :site_contact_username,
-    :site_contact_username=,
-    :site_contact_email,
-    :site_contact_email=,
-    :site_title,
-    :site_title=,
-    :site_short_description,
-    :site_short_description=,
-    :site_description,
-    :site_description=,
-    :site_extended_description,
-    :site_extended_description=,
-    :site_terms,
-    :site_terms=,
-    :registrations_mode,
-    :registrations_mode=,
-    :closed_registrations_message,
-    :closed_registrations_message=,
-    :open_deletion,
-    :open_deletion=,
-    :timeline_preview,
-    :timeline_preview=,
-    :show_staff_badge,
-    :show_staff_badge=,
-    :bootstrap_timeline_accounts,
-    :bootstrap_timeline_accounts=,
-    :theme,
-    :theme=,
-    :min_invite_role,
-    :min_invite_role=,
-    :activity_api_enabled,
-    :activity_api_enabled=,
-    :peers_api_enabled,
-    :peers_api_enabled=,
-    :show_known_fediverse_at_about_page,
-    :show_known_fediverse_at_about_page=,
-    :preview_sensitive_media,
-    :preview_sensitive_media=,
-    :custom_css,
-    :custom_css=,
-    :profile_directory,
-    :profile_directory=,
-    to: Setting
-  )
+  KEYS = %i(
+    site_contact_username
+    site_contact_email
+    site_title
+    site_short_description
+    site_description
+    site_extended_description
+    site_terms
+    registrations_mode
+    closed_registrations_message
+    open_deletion
+    timeline_preview
+    show_staff_badge
+    bootstrap_timeline_accounts
+    theme
+    min_invite_role
+    activity_api_enabled
+    peers_api_enabled
+    show_known_fediverse_at_about_page
+    preview_sensitive_media
+    custom_css
+    profile_directory
+  ).freeze
+
+  BOOLEAN_KEYS = %i(
+    open_deletion
+    timeline_preview
+    show_staff_badge
+    activity_api_enabled
+    peers_api_enabled
+    show_known_fediverse_at_about_page
+    preview_sensitive_media
+    profile_directory
+  ).freeze
+
+  UPLOAD_KEYS = %i(
+    thumbnail
+    hero
+    mascot
+  ).freeze
+
+  attr_accessor(*KEYS)
+
+  validates :site_short_description, :site_description, :site_extended_description, :site_terms, :closed_registrations_message, html: true
+  validates :registrations_mode, inclusion: { in: %w(open approved none) }
+  validates :min_invite_role, inclusion: { in: %w(disabled user moderator admin) }
+  validates :site_contact_email, :site_contact_username, presence: true
+  validates :site_contact_username, existing_username: true
+  validates :bootstrap_timeline_accounts, existing_username: { multiple: true }
+
+  def initialize(_attributes = {})
+    super
+    initialize_attributes
+  end
+
+  def save
+    return false unless valid?
+
+    KEYS.each do |key|
+      value = instance_variable_get("@#{key}")
+
+      if UPLOAD_KEYS.include?(key)
+        upload = SiteUpload.where(var: key).first_or_initialize(var: key)
+        upload.update(file: value)
+      else
+        setting = Setting.where(var: key).first_or_initialize(var: key)
+        setting.update(value: typecast_value(key, value))
+      end
+    end
+  end
+
+  private
+
+  def initialize_attributes
+    KEYS.each do |key|
+      instance_variable_set("@#{key}", Setting.public_send(key)) if instance_variable_get("@#{key}").nil?
+    end
+  end
+
+  def typecast_value(key, value)
+    if BOOLEAN_KEYS.include?(key)
+      value == '1'
+    else
+      value
+    end
+  end
 end
diff --git a/app/validators/existing_username_validator.rb b/app/validators/existing_username_validator.rb
new file mode 100644
index 0000000000..4388a0c983
--- /dev/null
+++ b/app/validators/existing_username_validator.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class ExistingUsernameValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    return if value.blank?
+
+    if options[:multiple]
+      missing_usernames = value.split(',').map { |username| username unless Account.find_local(username) }.compact
+      record.errors.add(attribute, I18n.t('existing_username_validator.not_found_multiple', usernames: missing_usernames.join(', '))) if missing_usernames.any?
+    else
+      record.errors.add(attribute, I18n.t('existing_username_validator.not_found')) unless Account.find_local(value)
+    end
+  end
+
+  private
+
+  def valid_html?(str)
+    Nokogiri::HTML.fragment(str).to_s == str
+  end
+end
diff --git a/app/validators/html_validator.rb b/app/validators/html_validator.rb
new file mode 100644
index 0000000000..882c35d413
--- /dev/null
+++ b/app/validators/html_validator.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class HtmlValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    return if value.blank?
+    record.errors.add(attribute, I18n.t('html_validator.invalid_markup')) unless valid_html?(value)
+  end
+
+  private
+
+  def valid_html?(str)
+    Nokogiri::HTML.fragment(str).to_s == str
+  end
+end
diff --git a/app/views/admin/settings/edit.html.haml b/app/views/admin/settings/edit.html.haml
index d9b4bf01b8..1c2c00f109 100644
--- a/app/views/admin/settings/edit.html.haml
+++ b/app/views/admin/settings/edit.html.haml
@@ -2,6 +2,7 @@
   = t('admin.settings.title')
 
 = simple_form_for @admin_settings, url: admin_settings_path, html: { method: :patch } do |f|
+  = render 'shared/error_messages', object: @admin_settings
 
   .fields-group
     = f.input :site_title, wrapper: :with_label, label: t('admin.settings.site_title')
diff --git a/config/locales/en.yml b/config/locales/en.yml
index ba42e7ce1b..d5ed206238 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -586,6 +586,9 @@ en:
       content: We're sorry, but something went wrong on our end.
       title: This page is not correct
     noscript_html: To use the Mastodon web application, please enable JavaScript. Alternatively, try one of the <a href="%{apps_path}">native apps</a> for Mastodon for your platform.
+  existing_username_validator:
+    not_found: could not find a local user with that username
+    not_found_multiple: could not find %{usernames}
   exports:
     archive_takeout:
       date: Date
@@ -633,6 +636,8 @@ en:
     validation_errors:
       one: Something isn't quite right yet! Please review the error below
       other: Something isn't quite right yet! Please review %{count} errors below
+  html_validator:
+    invalid_markup: contains invalid HTML markup
   identity_proofs:
     active: Active
     authorize: Yes, authorize
diff --git a/config/navigation.rb b/config/navigation.rb
index 07aec4b9de..f136141b3a 100644
--- a/config/navigation.rb
+++ b/config/navigation.rb
@@ -37,7 +37,7 @@ SimpleNavigation::Configuration.run do |navigation|
 
     primary.item :admin, safe_join([fa_icon('cogs fw'), t('admin.title')]), admin_dashboard_url, if: proc { current_user.staff? } do |admin|
       admin.item :dashboard, safe_join([fa_icon('tachometer fw'), t('admin.dashboard.title')]), admin_dashboard_url
-      admin.item :settings, safe_join([fa_icon('cogs fw'), t('admin.settings.title')]), edit_admin_settings_url, if: -> { current_user.admin? }
+      admin.item :settings, safe_join([fa_icon('cogs fw'), t('admin.settings.title')]), edit_admin_settings_url, if: -> { current_user.admin? }, highlights_on: %r{/admin/settings}
       admin.item :custom_emojis, safe_join([fa_icon('smile-o fw'), t('admin.custom_emojis.title')]), admin_custom_emojis_url, highlights_on: %r{/admin/custom_emojis}
       admin.item :relays, safe_join([fa_icon('exchange fw'), t('admin.relays.title')]), admin_relays_url, if: -> { current_user.admin? }, highlights_on: %r{/admin/relays}
       admin.item :subscriptions, safe_join([fa_icon('paper-plane-o fw'), t('admin.subscriptions.title')]), admin_subscriptions_url, if: -> { current_user.admin? }
diff --git a/spec/controllers/admin/settings_controller_spec.rb b/spec/controllers/admin/settings_controller_spec.rb
index 34f6bbdae0..6cf0ee20a6 100644
--- a/spec/controllers/admin/settings_controller_spec.rb
+++ b/spec/controllers/admin/settings_controller_spec.rb
@@ -19,6 +19,10 @@ RSpec.describe Admin::SettingsController, type: :controller do
     end
 
     describe 'PUT #update' do
+      before do
+        allow_any_instance_of(Form::AdminSettings).to receive(:valid?).and_return(true)
+      end
+
       describe 'for a record that doesnt exist' do
         around do |example|
           before = Setting.site_extended_description