45 files declare their use of MarkupInterface in Drupal 8

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