Issue #3268306 by lauriii, Wim Leers, catch, Dom.: [GHS] Custom/unofficial...

    // Add a node with text rendered via the Plain Text format.

    $this->drupalGet('node/add');

    $page->fillField('title[0][value]', 'My test content');

    $page->fillField('body[0][value]', '<p><a style="color:#ff0000;" foo="bar" hreflang="en" href="https://example.com"><abbr title="National Aeronautics and Space Administration">NASA</abbr> is an acronym.</a></p>');

    $page->fillField('body[0][value]', '<foo bar="baz">⬅️✌️➡️</foo><p><a style="color:#ff0000;" foo="bar" hreflang="en" href="https://example.com"><abbr title="National Aeronautics and Space Administration">NASA</abbr> is an acronym.</a></p>');

    $page->pressButton('Save');

    // Configure Full HTML text format to use CKEditor 5.

    // But note that the `style` attribute was stripped by

    // \Drupal\editor\EditorXssFilter\Standard.

    $assert_session->responseContains('<p><a foo="bar" hreflang="en" href="https://example.com"><abbr title="National Aeronautics and Space Administration">NASA</abbr> is an acronym.</a></p>');

    $assert_session->responseContains('<foo bar="baz">⬅️✌️➡️</foo><p><a foo="bar" hreflang="en" href="https://example.com"><abbr title="National Aeronautics and Space Administration">NASA</abbr> is an acronym.</a></p>');

    // Ensure attributes are retained after enabling link plugin.

    $this->drupalGet('admin/config/content/formats/manage/full_html');

    $page->pressButton('Save');

    // The `style` and foo` attributes should have been removed, as should the

    // `<abbr>` tag.

    $assert_session->responseContains('<p><a href="https://example.com" hreflang="en">NASA is an acronym.</a></p>');

    // `<abbr>` and `<foo>` tags.

    $assert_session->responseContains('<p>⬅️✌️➡️</p><p><a href="https://example.com" hreflang="en">NASA is an acronym.</a></p>');

  }

}