Experiment: Improvements for the Image block lightbox

[afercia]

Description

Follow-up to #50373

While testing the Image block lightbox, noticed a few things that can be improved. Main issues:

• Some strings are not translatable. This was already mentioned in the original PR>
• Translatable strings must avoid concatenation. Use full sentences instead.
• When the image has an empty alt text, the modal dialog is unlabelled (because aria-labelledby points to the image with the empty alt text).
• Missing escaping for some HTML attributes.
• Potentially duplicate IDs when hte same image is added more than once on a page. This would also cause incorrect labelling of the modal dialog.
• The buttons miss type="button". I'm not sure the buttons default action is prevented. There is a potential risk to submit a form in the edge case where the lightbox button is rendered within a form.

Step-by-step reproduction instructions

• Check the code at https://github.com/WordPress/gutenberg/blob/2b687d03ecdfb52229c25d5d455038b14fc8fa89/packages/block-library/src/image/index.php and see the button labels are not translatable:
  • $aria_label = 'Open image lightbox';
  • aria-label="Close lightbox"
• Observe one translatable string uses concatenation.
• Make sure to enable Core blocks in the Gutenberg Experiments settings page.
• Add an image with no alt text.
• Set the image to open in a lightbox (from the Advanced settings in the block settings sidebar).
• Publisha and go to the fron end.
• Open the image lightbox by clicking the image.
• Inspect the source and check the element with role=dialog has an aria-labelledby attribute that points to the ID of the image with the empty alt text. As such, the dialog is unlabelled.
• Inspect the source and observe the buttosn to open and close the lightbox miss a type="button" attribute.

Screenshots, screen recording, code snippet

No response

Environment info

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Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes