45 files declare their use of MarkupInterface

AssertHelperTrait.php in core/tests/Drupal/Tests/AssertHelperTrait.php
AssertLegacyTraitTest.php in core/tests/Drupal/Tests/Core/Assert/AssertLegacyTraitTest.php
Attribute.php in core/lib/Drupal/Core/Template/Attribute.php
AttributeTest.php in core/tests/Drupal/Tests/Core/Template/AttributeTest.php
AuditResult.php in core/modules/migrate/src/Audit/AuditResult.php
BigPipeMarkup.php in core/modules/big_pipe/src/Render/BigPipeMarkup.php
CommentPreviewTest.php in core/modules/comment/tests/src/Functional/CommentPreviewTest.php
ConfigBase.php in core/lib/Drupal/Core/Config/ConfigBase.php
DrupalTranslator.php in core/lib/Drupal/Core/Validation/DrupalTranslator.php
FieldApiDataTest.php in core/modules/views/tests/src/Kernel/FieldApiDataTest.php
FieldFilteredMarkup.php in core/lib/Drupal/Core/Field/FieldFilteredMarkup.php
FieldFilteredMarkupTest.php in core/tests/Drupal/Tests/Core/Field/FieldFilteredMarkupTest.php
FieldPluginBase.php in core/modules/views/src/Plugin/views/field/FieldPluginBase.php
FilteredMarkup.php in core/modules/filter/src/Render/FilteredMarkup.php
GeneratedLink.php in core/lib/Drupal/Core/GeneratedLink.php
HtmlEscapedTextTest.php in core/tests/Drupal/Tests/Component/Render/HtmlEscapedTextTest.php
HtmlTag.php in core/lib/Drupal/Core/Render/Element/HtmlTag.php
HtmlTest.php in core/tests/Drupal/Tests/Component/Utility/HtmlTest.php
IFrameMarkup.php in core/modules/media/src/IFrameMarkup.php
InOperator.php in core/modules/views/src/Plugin/views/filter/InOperator.php
LegacyMessenger.php in core/lib/Drupal/Core/Messenger/LegacyMessenger.php
LinkGenerationTest.php in core/tests/Drupal/KernelTests/Core/Url/LinkGenerationTest.php
LinkGenerator.php in core/lib/Drupal/Core/Utility/LinkGenerator.php
LinkGeneratorTest.php in core/tests/Drupal/Tests/Core/Utility/LinkGeneratorTest.php
MailManager.php in core/lib/Drupal/Core/Mail/MailManager.php
Markup.php in core/lib/Drupal/Core/Render/Markup.php
MarkupInterfaceComparator.php in core/tests/Drupal/TestTools/Comparator/MarkupInterfaceComparator.php
MarkupNormalizer.php in core/modules/serialization/src/Normalizer/MarkupNormalizer.php
Messenger.php in core/lib/Drupal/Core/Messenger/Messenger.php
PlainTextOutputTest.php in core/tests/Drupal/Tests/Component/Render/PlainTextOutputTest.php
PrimitiveTypeConstraintValidator.php in core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/PrimitiveTypeConstraintValidator.php
Renderer.php in core/lib/Drupal/Core/Render/Renderer.php
RendererTest.php in core/tests/Drupal/Tests/Core/Render/RendererTest.php
Contains \Drupal\Tests\Core\Render\RendererTest.
SafeMarkup.php in core/lib/Drupal/Component/Utility/SafeMarkup.php
SafeMarkupTest.php in core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
Contains \Drupal\Tests\Component\Utility\SafeMarkupTest.
TestBase.php in core/modules/simpletest/src/TestBase.php
theme.inc in core/includes/theme.inc
The theme system, which controls the output of Drupal.
ThemeManager.php in core/lib/Drupal/Core/Theme/ThemeManager.php
ThemeTest.php in core/modules/system/tests/src/Kernel/Theme/ThemeTest.php
Token.php in core/lib/Drupal/Core/Utility/Token.php
TranslationManagerTest.php in core/tests/Drupal/Tests/Core/StringTranslation/TranslationManagerTest.php
Contains \Drupal\Tests\Core\StringTranslation\TranslationManagerTest.
TwigExtension.php in core/lib/Drupal/Core/Template/TwigExtension.php
TwigMarkupInterfaceTest.php in core/tests/Drupal/KernelTests/Core/Theme/TwigMarkupInterfaceTest.php
Contains \Drupal\KernelTests\Core\Theme\TwigMarkupInterfaceTest.
ViewsFormMainForm.php in core/modules/views/src/Form/ViewsFormMainForm.php
ViewsRenderPipelineMarkup.php in core/modules/views/src/Render/ViewsRenderPipelineMarkup.php

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