Block Data Attribute

Open a page or post in the block editor. Select a supported block (e.g. a Paragraph or Button block). Open the block settings sidebar by clicking the ⚙ Settings icon in the top-right toolbar. Scroll down to the Block Data Attribute panel and expand it. Fill in the Name and Value fields. The name will automatically be prefixed with data- in the saved HTML. Click + Add attribute to add more pairs as needed. Save or publish your post.

A paragraph block with a data-highlight="true" attribute would produce:

<p data-highlight="true">Your content here.</p>

By default: core/button , core/column , core/columns , core/group , core/heading , and core/paragraph .

Yes. Use the mypreview.blockDataAttributeAllowedBlocks JavaScript filter:

wp.hooks.addFilter( 'mypreview.blockDataAttributeAllowedBlocks', 'my-plugin/extend-allowed-blocks', ( allowedBlocks ) => [ ...allowedBlocks, 'core/image', 'core/list' ] );

No. Attributes are stored as part of the block’s serialised markup. The plugin uses the blocks.getSaveContent.extraProps filter, which is the officially supported WordPress API for this purpose.

Yes. The plugin works in any context where the block editor is used, including the Site Editor.