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Hundreds of changes were made between Vanilla 2.2. and 2.3. Here is a highlight reel:
Add HTML email support.
Redesign Applicants approval screen.
Add spoilers functionality to core.
Add default avatar feature.
Add NBBC (New BBCode) support to core.
Add support for Imgur and Wistia.
Add discussions sort & filter module for developers.
Add garden-http library as core dependency.
Upgrade to reCaptcha2 ("NoCaptcha").
Improve 404 & error handling.
Add new dropdown menu handling.
Improve mention & username validation handling.
Fix GIF upload rendering in Editor.
Improve the Debugger addon for developers.
Fix stashing of comment drafts by guests.
Add CivilTongue support for private messages.
Remove jQuery Livequery and dependencies on it.
Add CORS support for trusted domains.
Fix port handling issues.
Alter dispatching to properly support PATH_INFO.
Change default input formatter to Markdown.
Improve role API endpoints.
Improve advanced embedding system.
Restore user count to default user list for medium-sized sites.
Add Phing support to builds & releases (so meta) for developers.
Add Travis-CI and PHPUnit integrations for developers.
Allow user photo to be set over API.
Fix support for MySQL strict mode in multiple places.
Eliminate Garden.Charset (Vanilla is UTF-8 only).
Remove base64-encoding of JSON data.
Optimize browser caching headers.
Improve SSO connection workflow.
Implement and alter table threshold to prevent massive db changes in production.
Remove gender from registration form.
Implement PSR-4 autoloading (first steps).
Multiple formatting fixes to Quotes, Markdown, and BBCode.
Fix hundreds of minor issues.
Now supports PHP 7.0.