I try to modify the gallery block of Gutenberg and I use the following snippet

export function defaultColumnsNumber( attributes ) {
    return Math.min( 3, attributes.images.length );

function modifyGalleryMarkup( element, blockType, attributes ) {

    if ( blockType.name !== 'core/gallery' ) {
        return element;

    const { images, columns = defaultColumnsNumber( attributes ), imageCrop, linkTo } = attributes;

    var newElement = wp.element.createElement(
            'className': `lightbox columns-${ columns } ${ imageCrop ? 'is-cropped' : '' }`,
            function( image ) {
                return wp.element.createElement(
                        'className': 'blocks-gallery-item my-gallery-item',
                            'src': image.url,

    return newElement


I can save it but on page reload I get the following error in console

Block validation failed for `core/gallery`


<ul class="lightbox columns-0 is-cropped"></ul>


<ul class="lightbox columns-3 is-cropped">< ....

what I'm missing in my snippet?

1 Answer 1


Seems that when you modify the existing block's HTML, then it is no longer being validated. Instead, you might try to create your own block. Look here is a good approach and here too.

  • 1
    I made a custom gallery block based on the source code, but taking a consideration the effort I would say it is bit too much. Would be great to can modify some markup attributes on the flow of the core blocks.
    – fefe
    Jan 13, 2019 at 20:57

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