63 files declare their use of Element

AccountSettingsForm.php in core/modules/user/src/AccountSettingsForm.php
Actions.php in core/lib/Drupal/Core/Render/Element/Actions.php
BlockViewBuilder.php in core/modules/block/src/BlockViewBuilder.php
Button.php in core/lib/Drupal/Core/Render/Element/Button.php
Checkbox.php in core/lib/Drupal/Core/Render/Element/Checkbox.php
Color.php in core/lib/Drupal/Core/Render/Element/Color.php
comment.module in core/modules/comment/comment.module
Enables users to comment on published content.
ComponentElement.php in core/lib/Drupal/Core/Render/Element/ComponentElement.php
ComponentValidator.php in core/lib/Drupal/Core/Theme/Component/ComponentValidator.php
ConfigFormBase.php in core/lib/Drupal/Core/Form/ConfigFormBase.php
Container.php in core/lib/Drupal/Core/Render/Element/Container.php
Date.php in core/lib/Drupal/Core/Render/Element/Date.php
Details.php in core/lib/Drupal/Core/Render/Element/Details.php
editor.module in core/modules/editor/editor.module
Adds bindings for client-side "text editors" to text formats.
ElementTest.php in core/tests/Drupal/Tests/Core/Render/ElementTest.php
Email.php in core/lib/Drupal/Core/Render/Element/Email.php
EntityForm.php in core/lib/Drupal/Core/Entity/EntityForm.php
EntityReferenceItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
EntityViewBuilder.php in core/lib/Drupal/Core/Entity/EntityViewBuilder.php
EntityViewDisplay.php in core/lib/Drupal/Core/Entity/Entity/EntityViewDisplay.php
EntityViewTrait.php in core/tests/Drupal/Tests/EntityViewTrait.php
File.php in core/lib/Drupal/Core/Render/Element/File.php
FileTransferAuthorizeForm.php in core/lib/Drupal/Core/FileTransfer/Form/FileTransferAuthorizeForm.php
form.inc in core/includes/form.inc
Functions for form and batch generation and processing.
FormatterBase.php in core/lib/Drupal/Core/Field/FormatterBase.php
FormBuilder.php in core/lib/Drupal/Core/Form/FormBuilder.php
FormElementHelper.php in core/lib/Drupal/Core/Form/FormElementHelper.php
FormErrorHandler.php in core/lib/Drupal/Core/Form/FormErrorHandler.php
FormHelper.php in core/lib/Drupal/Core/Form/FormHelper.php
FormTest.php in core/modules/system/tests/src/Functional/Form/FormTest.php
FormValidator.php in core/lib/Drupal/Core/Form/FormValidator.php
Hidden.php in core/lib/Drupal/Core/Render/Element/Hidden.php
image.admin.inc in core/modules/image/image.admin.inc
Administration pages for image settings.
image.field.inc in core/modules/image/image.field.inc
Implement an image field, based on the file module's file field.
ImageButton.php in core/lib/Drupal/Core/Render/Element/ImageButton.php
Link.php in core/lib/Drupal/Core/Render/Element/Link.php
LocalTasksBlock.php in core/lib/Drupal/Core/Menu/Plugin/Block/LocalTasksBlock.php
NavigationLayout.php in core/modules/navigation/src/NavigationLayout.php
NavigationMenuBlockTest.php in core/modules/navigation/tests/src/Kernel/NavigationMenuBlockTest.php
Number.php in core/lib/Drupal/Core/Render/Element/Number.php
Password.php in core/lib/Drupal/Core/Render/Element/Password.php
Radio.php in core/lib/Drupal/Core/Render/Element/Radio.php
Range.php in core/lib/Drupal/Core/Render/Element/Range.php
RenderElementBase.php in core/lib/Drupal/Core/Render/Element/RenderElementBase.php
RendererTest.php in core/tests/Drupal/Tests/Core/Render/RendererTest.php
Search.php in core/lib/Drupal/Core/Render/Element/Search.php
Select.php in core/lib/Drupal/Core/Render/Element/Select.php
system.admin.inc in core/modules/system/system.admin.inc
Admin page callbacks for the system module.
SystemMenuBlockTest.php in core/modules/system/tests/src/Kernel/Block/SystemMenuBlockTest.php
Table.php in core/lib/Drupal/Core/Render/Element/Table.php
Tableselect.php in core/lib/Drupal/Core/Render/Element/Tableselect.php
taxonomy.module in core/modules/taxonomy/taxonomy.module
Enables the organization of content into categories.
Tel.php in core/lib/Drupal/Core/Render/Element/Tel.php
Textfield.php in core/lib/Drupal/Core/Render/Element/Textfield.php
theme.inc in core/includes/theme.inc
The theme system, which controls the output of Drupal.
toolbar.module in core/modules/toolbar/toolbar.module
Administration toolbar for quick access to top level administration items.
Toolbar.php in core/modules/toolbar/src/Element/Toolbar.php
Url.php in core/lib/Drupal/Core/Render/Element/Url.php
user.module in core/modules/user/user.module
Enables the user registration and login system.
ValidationTest.php in core/modules/system/tests/src/Functional/Form/ValidationTest.php
VerticalTabs.php in core/lib/Drupal/Core/Render/Element/VerticalTabs.php
views_ui.theme.inc in core/modules/views_ui/views_ui.theme.inc
Preprocessors and theme functions for the Views UI.
WidgetBase.php in core/lib/Drupal/Core/Field/WidgetBase.php

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