In WYSIWYG embed modal for existing images, the alt text field should have a default value instead of placeholder text

For maximum usability, I would suggest the following.

• Show the existing media library default alternative text outside the field, so that someone who is typing new text in the field can refer to the existing text while they are typing the new text. This reduces cognitive load and also makes it easier for the content editor to
  • decide whether they want to keep the default alt text or write new alt text.
  • revert any edits while still having the original available to them.
• Ensure that the displayed existing text is copy-and-paste-able.