// config/editorjs.php
return [
     //...
    'validationSettings' => [
        'tools' => [
            //...
        ],
        'customTags' => [
            // allow <dfn type="" slug="" class="" description=""></dfn>
            'dfn' => [
                'type' => 'Inline',
                'contents' => 'Inline',
                'collection' => 'Common',
                'attributes' => [
                    'type' => 'CDATA',
                    'slug' => 'CDATA',
                    'class' => 'CDATA',
                    'description' => 'CDATA',
                ],
            ],
        ],
    ],
];