With Development Mode, features that do not require a connection to WordPress.com servers can be activated on a localhost WordPress installation for testing.
You can turn on Development Mode by adding
define( 'JETPACK_DEV_DEBUG', true );
to your wp-config.php file. Make sure you add it before the /* That's all, stop editing! Happy blogging. */ line.
Development mode automatically gets enabled if you don’t have a period in your site’s hostname, i.e. localhost. If you use a different URL, such as mycooltestsite.local or something, then you will need to define the JETPACK_DEV_DEBUG constant.
You can also enable Jetpack’s development mode through a plugin, thanks to the jetpack_development_mode filter:
add_filter( 'jetpack_development_mode', '__return_true' );
While in Development Mode, some features will not be available at all as they require WordPress.com for all functionality—Related Posts and Publicize, for example. Other features will have reduced functionality to give developers a good-faith representation of the feature. For example, Tiled Galleries requires the WordPress.com Photon CDN; however, in Development Mode, Jetpack provides a fallback so developers can have a similar experience during development and testing.
