diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 782583e..2ec0aeb 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -55,9 +55,9 @@ class ApplicationController < ActionController::Base
 
   private
 
-  def render_no_content(record)
-    helpers.render_errors(record)
-    render html: nil, layout: true
+  def render_errors(record)
+    target = record.new_record? ? [:new, record.parent || record.class] : [:edit, record]
+    render 'errors', assigns: {record: record, target: target}
   end
 
   def rescue_turbo(exception)
diff --git a/app/controllers/quantities_controller.rb b/app/controllers/quantities_controller.rb
index 5c0c5ba..86dc12d 100644
--- a/app/controllers/quantities_controller.rb
+++ b/app/controllers/quantities_controller.rb
@@ -43,12 +43,15 @@ class QuantitiesController < ApplicationController
     permitted = params.require(:quantity).permit(:parent_id)
     @previous_ancestors = @quantity.ancestors
 
-    # Until UI blocks all disallowed reparents, render error messages if present
-    render_no_content(@quantity) unless @quantity.update(permitted)
-
-    @ancestors = @quantity.ancestors
-    @self_and_progenies = @quantity.with_progenies
-    @before = @self_and_progenies.last.successive
+    if @quantity.update(permitted)
+      @ancestors = @quantity.ancestors
+      @self_and_progenies = @quantity.with_progenies
+      @before = @self_and_progenies.last.successive
+    else
+      # Until UI blocks all disallowed reparents, render error messages if present.
+      flash.now.alert = @quantity.errors.full_messages
+      render :nothing
+    end
   end
 
   def destroy
diff --git a/app/controllers/units_controller.rb b/app/controllers/units_controller.rb
index c876cbc..b064773 100644
--- a/app/controllers/units_controller.rb
+++ b/app/controllers/units_controller.rb
@@ -22,7 +22,7 @@ class UnitsController < ApplicationController
       @before = @unit.successive
       flash.now[:notice] = t('.success', unit: @unit)
     else
-      render :new
+      render_errors @unit
     end
   end
 
@@ -33,7 +33,7 @@ class UnitsController < ApplicationController
     if @unit.update(params.except(:base_id).expect(Unit::ATTRIBUTES))
       flash.now[:notice] = t('.success', unit: @unit)
     else
-      render :edit
+      render_errors @unit
     end
   end
 
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index a903eee..f997d0b 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -91,10 +91,7 @@ module ApplicationHelper
     [:text_field, :password_field, :text_area].each do |selector|
       class_eval <<-RUBY_EVAL, __FILE__, __LINE__ + 1
         def #{selector}(method, options = {})
-          options[:maxlength] ||= object.class.type_for_attribute(method).limit
-          if object.errors.include?(method)
-            options[:pattern] = except_pattern(object.public_send(method), options[:pattern])
-          end
+          options[:maxlength] ||= object.type_for_attribute(method).limit
           super
         end
       RUBY_EVAL
@@ -129,10 +126,6 @@ module ApplicationHelper
       svg_name = object ? (object.persisted? ? 'update' : 'plus-circle-outline') : ''
       @template.svg_tag(svg_name, super)
     end
-
-    def except_pattern(value, pattern = nil)
-      "(?!^#{Regexp.escape(value)}$)" + (pattern || ".*")
-    end
   end
 
   def tabular_fields_for(record_name, record_object = nil, options = {}, &block)
@@ -142,8 +135,6 @@ module ApplicationHelper
     record_object, options = nil, record_object if record_object.is_a?(Hash)
     extra_options = {builder: TabularFormBuilder, skip_default_ids: true}
     options = merge_attributes(options, extra_options)
-    # TODO: set error message with setCustomValidity instead of rendering to flash?
-    render_errors(record_object || record_name)
     fields_for(record_name, record_object, **options, &block)
   end
 
@@ -210,16 +201,9 @@ module ApplicationHelper
     link_to name, options, html_options
   end
 
-  def render_errors(records)
-    # Conversion of flash to Array only required because of Devise
-    # TODO: override Devise message setting to Array()?
-    flash[:alert] = Array(flash[:alert])
-    Array(records).each { |record| flash[:alert] += record.errors.full_messages }
-  end
-
-  def render_flash_messages
+  def render_flash
     flash.map do |entry, messages|
-      # Conversion of flash to Array only required because of Devise
+      # Conversion of flash to Array only required because of Devise.
       Array(messages).map do |message|
         tag.div class: "flash #{entry}" do
           tag.span(sanitize(message)) +
diff --git a/app/javascript/application.js b/app/javascript/application.js
index a679411..230a5de 100644
--- a/app/javascript/application.js
+++ b/app/javascript/application.js
@@ -1,6 +1,8 @@
 // Configure your import map in config/importmap.rb. Read more:
 // https://github.com/rails/importmap-rails
 import "@hotwired/turbo-rails"
+// TODO: access `data-` attributes through
+// `keyname in`/`dataset.keyname`/`delete dataset.keyname`
 
 
 // Show page if hidden for testing purposes.
@@ -146,6 +148,33 @@ Turbo.StreamActions.unselect = function() {
   })
 }
 
+Turbo.StreamActions.validate = function() {
+  this.targetElements.forEach((form) => {
+    this.templateContent.querySelectorAll("span").forEach((span) => {
+      var control = document.querySelector(
+        `[form="${form.id}"][name="${span.dataset.name}"]`
+      )
+      control.setCustomValidity(span.textContent)
+      // Add event listener in capture phase to clear custom validity before
+      // `[oninput]` or any other bubble phase handler is invoked.
+      //   https://www.quirksmode.org/js/events_order.html
+      // TODO: keep invalidating current value whenever user re-enters it. Store
+      // invalid value and validation message in `[data-invalid-message/value]`
+      // and check input value on "input" event (below) (using `[pattern]` may not
+      // be suitable due to inability to customize validtion message; also
+      // pattern may be used for other, conflicting purposes).
+      // After user executes any other action (while form is open), remove `[data-]`
+      // attributes and custom validation, as value may no longer be invalid.
+      control.addEventListener(
+        "input",
+        function(event) { event.target.setCustomValidity("") },
+        {capture: true, once: true}
+      )
+    })
+    form.reportValidity()
+  })
+}
+
 
 // Keyboard event processing handlers.
 function formProcessKey(event) {
@@ -209,6 +238,8 @@ window.dragStart = dragStart
 *     rapid pointer moves.
 * NOTE: sometimes Leave is not emitted when pointer goes fast over table
 * and outside. This should probably be fixed in browser, than patched here.
+* TODO: replace Leave table with Enter document?
+*   https://www.quirksmode.org/js/events_order.html#link4
 */
 function dragEnter(event) {
   //console.log(event.timeStamp + " " + event.type + ": " + event.currentTarget.id)
diff --git a/app/models/unit.rb b/app/models/unit.rb
index 597769d..bc82353 100644
--- a/app/models/unit.rb
+++ b/app/models/unit.rb
@@ -3,6 +3,7 @@ class Unit < ApplicationRecord
 
   belongs_to :user, optional: true
   belongs_to :base, optional: true, class_name: "Unit"
+  alias_method :parent, :base
   has_many :subunits, class_name: "Unit", inverse_of: :base,
     dependent: :restrict_with_error
 
diff --git a/app/views/application/errors.turbo_stream.erb b/app/views/application/errors.turbo_stream.erb
new file mode 100644
index 0000000..e955533
--- /dev/null
+++ b/app/views/application/errors.turbo_stream.erb
@@ -0,0 +1,7 @@
+<%= turbo_stream.validate dom_target(*@target, :form) do %>
+  <% @record.errors.attribute_names.map do |attr| %>
+    <%= tag.span data: {name: field_name(dom_class(@record), attr)} do %>
+      <%- @record.errors.full_messages_for(attr).map { |m| m + '.' }.join(' ') %>
+    <% end %>
+  <% end %>
+<% end %>
diff --git a/app/views/application/nothing.turbo_stream.erb b/app/views/application/nothing.turbo_stream.erb
new file mode 100644
index 0000000..e69de29
diff --git a/app/views/layouts/application.html.erb b/app/views/layouts/application.html.erb
index 6167976..4fbe58e 100644
--- a/app/views/layouts/application.html.erb
+++ b/app/views/layouts/application.html.erb
@@ -44,7 +44,7 @@
     </header>
 
     <div id="flashes">
-      <%= render_flash_messages %>
+      <%= render_flash %>
     </div>
 
     <%# Allows overwriting/clearing navigation menu for some views %>
diff --git a/app/views/layouts/application.turbo_stream.erb b/app/views/layouts/application.turbo_stream.erb
index 9ba0c29..1312da1 100644
--- a/app/views/layouts/application.turbo_stream.erb
+++ b/app/views/layouts/application.turbo_stream.erb
@@ -1,6 +1,6 @@
-<%= yield %>
-
-<%# Some views may convert ActiveRecord errors to flashes, render at the end. %>
+<%# Render unconditionally, to clear previous flash messages. %>
 <%= turbo_stream.update :flashes do %>
-  <%= render_flash_messages %>
+  <%= render_flash %>
 <% end %>
+
+<%= yield %>
diff --git a/app/views/users/passwords/create.turbo_stream.erb b/app/views/users/passwords/create.turbo_stream.erb
index 99acbf4..6e7b33a 100644
--- a/app/views/users/passwords/create.turbo_stream.erb
+++ b/app/views/users/passwords/create.turbo_stream.erb
@@ -1,2 +1 @@
-<%# For some reason flash messages are duplicated in bot flash and flash.now %>
 <% flash.discard %>
diff --git a/config/initializers/turbo_streams_tag_builder.rb b/config/initializers/turbo_streams_tag_builder.rb
index 70fc9c1..b104a62 100644
--- a/config/initializers/turbo_streams_tag_builder.rb
+++ b/config/initializers/turbo_streams_tag_builder.rb
@@ -34,4 +34,8 @@ ActiveSupport.on_load :turbo_streams_tag_builder do
   def unselect(target)
     action :unselect, target, allow_inferred_rendering: false
   end
+
+  def validate(target, content = nil, **rendering, &block)
+    action :validate, target, content, **rendering, &block
+  end
 end
diff --git a/test/system/units_test.rb b/test/system/units_test.rb
index 54c92dd..ccc6c6f 100644
--- a/test/system/units_test.rb
+++ b/test/system/units_test.rb
@@ -163,31 +163,32 @@ class UnitsTest < ApplicationSystemTestCase
     link = all(:link, exact_text: label).sample
     link.click
 
-    get_values = ->{ all(:field).map { |f| [f[:name], f.value] }.to_h }
-    values = nil
-    within :table_row, {}, with_focus: true do
-      # Provide duplicate :symbol as server-side invalidated input.
-      field = find(:table_cell, column(:symbol)).find(:fillable_field)
-      field.fill_in with: (symbols - [field.value]).sample
-      values = get_values[]
-      send_keys :enter
-    end
+    get_values = ->{
+      find(:table_row, {}, with_focus: true)
+        .all(:field).map { |f| [f[:name], f.value] }.to_h
+    }
 
-    assert_selector '.flash.alert',
-      text: t('activerecord.errors.models.unit.attributes.symbol.taken')
+    # Provide duplicate :symbol as server-side invalidated input.
+    field = find(:field, focused: true)
+    field.fill_in with: (symbols - [field.value]).sample
+    values = get_values[]
+    send_keys :enter
+
+    assert_matches_selector field, :field, validation_message:
+      /#{t('activerecord.errors.models.unit.attributes.symbol.taken')}/
+    assert_no_selector '.flash.alert'
+    assert_equal values, get_values[]
     if action == :edit
       assert_no_selector :link, exact_text: link[:text]
     else
       assert_matches_selector link, :link, disabled: true
     end
+    click_on t(:cancel)
 
-    within :table_row, {}, with_focus: true do
-      assert_equal values, get_values[]
-      click_on t(:cancel)
-    end
     assert_no_selector '.flash.alert'
     assert_equal symbols, list_symbols
     refresh
+    assert_no_selector '.flash.alert'
     assert_equal symbols, list_symbols
   end