Since WordPress 5.9 you can load assets for a block only when the block is used. For block styles/variations, you need to be creative, though. Especially if it’s not your own block you’re creating a variant of.

Since block variations are usually registered separately, and especially if you want to have it clean in a separate file or trying to extend an existing block, you cannot properly rely on something like register_block_type to register your assets with the existing block. This is because this would need to overwriting the render_callback property of said function and thus may break the block itself, since the original callback won’t be called anymore.

In my solution I use the render_block_{$this->name} hook, where it’s possible since quite some time to allow registering assets via wp_enqueue_script or wp_enqueue_style.

To enqueue assets only for a block on certain conditions, I use the following code:

static function my_group_callback( string $block_content, array $block ): string {
    if ( ! $condition ) { // check your condition
        return $block_content;
    \wp_enqueue_style( 'my-handle', $uri ); // adjust handle and uri
    return $block_content;

\add_action( 'render_block_core/group', 'my_group_callback', 10, 2 );
Code language: PHP (php)

In this example, I enqueue a style if a certain condition is met in a group block. This code is a boilerplate, since $condition and $uri are not defined here. The real code checks for the className attribute in $block['attrs']['className'] for a specific class that is added if my registered block variation is used. Only then the style is enqueued and loaded in the frontend.

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