8 files declare their use of MarkupTrait
- BigPipeMarkup.php in core/modules/ big_pipe/ src/ Render/ BigPipeMarkup.php 
- FieldFilteredMarkup.php in core/lib/ Drupal/ Core/ Field/ FieldFilteredMarkup.php 
- FilteredMarkup.php in core/modules/ filter/ src/ Render/ FilteredMarkup.php 
- HtmlTest.php in core/tests/ Drupal/ Tests/ Component/ Utility/ HtmlTest.php 
- IFrameMarkup.php in core/modules/ media/ src/ IFrameMarkup.php 
- Markup.php in core/lib/ Drupal/ Core/ Render/ Markup.php 
- TwigMarkupInterfaceTest.php in core/tests/ Drupal/ KernelTests/ Core/ Theme/ TwigMarkupInterfaceTest.php 
- Contains \Drupal\KernelTests\Core\Theme\TwigMarkupInterfaceTest.
- ViewsRenderPipelineMarkup.php in core/modules/ views/ src/ Render/ ViewsRenderPipelineMarkup.php 
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