Image missing alternative text can be inserted without error

Problem/Motivation

If an image has been added to the media library without alternative text, then subsequent attempts to insert that image into page content do not force supplying the missing alternative text.

I recognize that this would not normally happen on a manual image upload in the default Drupal configuration, but it could happen inadvertently as a result of a data upload or a temporary site misconfiguration.

Steps to reproduce

These steps are intended to create a testable case with minimal effort, so testers don't have to intentionally create a faulty data upload.

1. Go to simplytest.me
2. In the form field, type core
3. Choose Drupal core
4. Click to create the website with the latest version of Drupal core
5. Once the site is created, login as admin
6. In the administrator menu, click Extend
7. In the form field, type media
8. Enable Media and Media Library
9. Click Configure
10. Choose text formats and editors
11. On the row for basic HTML click Configure
12. Drag the media icon from Available Buttons to the Active Toolbar
13. Under Enabled filters, check Embed media
14. Under Filter settings, click Embed media
15. Check Image
16. Click Save configuration
17. In the administrator menu, click Structure
18. Click Media types
19. On the row for Image, choose Manage fields from the drop-down
20. On the row for image, click Edit
21. Uncheck Alt field required
22. Click save settings
23. In the administrator menu, click Content
24. Click Add content
25. Click Basic page
26. Click in the body WYSIWYG
27. Click the media icon
28. Click Choose files
29. Navigate to a file and click Upload
30. Do not type alternative text for the image
31. Click save
32. Click the X in the upper right corner to close the Add or save media dialog
33. In the administrator menu, click Structure
34. Click Media types
35. On the row for Image, choose Manage fields from the drop-down
36. On the row for Image, click Edit
37. Check Alt field required
38. Click save settings
39. In the administrator menu, click Content
40. Click Add content
41. Click Basic page
42. Click in the body WYSIWYG
43. Click the media icon
44. Check the checkbox for the image you just uploaded

    Option one:
    Expected result: A dialog box opens containing the Alternative text field, which is highlighted as being required.
    Actual result: Nothing happens.

    This is the issue filer's preferred option because it solves the issue for all future uses of this image.

45. Click insert selected
46. Type a page title
47. Click save

    Option two:
    Expected result: Get error message that the image is missing alternative text. The editor would need to use the slashed eyeball icon to add the alternative text for that image on this page only.
    Actual result: Nothing happens.

    This is the issue filer's second choice because it solves the issue for this page only.

I would expect a similar solution for an image field that uses the media library.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet