.visually-hidden not respected in CKEditor preview

Problem/Motivation

Over at #2998005-19: [PP-1] Support Drupal core's Media Library I discovered that a typical Media entity embed still has a label showing up. This label has class="visually-hidden", but the corresponding core CSS is not loaded in the CKEditor iframe and hence the label ends up being visible to content authors.

Proposed resolution

Load relevant core CSS in CKEditor iframe.

Remaining tasks

User interface changes

.visually-hidden content is no longer visible in CKEditor.

Before

After

API changes

None.

Data model changes

None.

Release notes snippet