Conversation System: Post new comments with AJAX

[vladolaru]

The default core experience of going to a separate page in case of errors, or reloading the page is not smooth enough. It is good as a fallback, but the primary experience should be AJAX-based, including validation and error messages. This should not mean duplicating the server-side logic, but only relying on it and presenting its results without page reload.

The WP Ajaxify Comments plugin is a good starting point.

Note: Related to #268