The library is at a stage in which the major functionality of the H5P editor and player are working. You can check in the lists below, what is already implemented and what isn't.
Finished functionality
store and serve libraries / content types
store and serve content
create, read, update and delete operations on content
manage file uploads (= images, video etc.) in temporary files
decoupling of storage through interfaces
provide AJAX endpoints for the editor and player
backend communication with the H5P Hub
register the site
send usage statistics
get information about content types on the hub
download and install new content types / updates of content types on user request
validation of packages (structural integrity and conformity of content and libraries)
offers downloads for h5p packages ("exporting" content)
support for copy & paste in the editor
support for editor interface languages other than English
addons (required to display mathematical formulas)
MongoDB and S3 storage implementation for content and temporary files
library administration endpoint and React UI component
MongoDB and S3 storage implementation for libraries
Redis cache for caching
catch and relay xAPI statements
alter library files, semantics (allows site admins to change libraries without hacking the actual files; very useful) (published soon)
filter html to prevent cross-site-scripting (XSS) vulnerabilities.
add csrf tokens to AJAX POST requests
Unfinished functionality
storing user state in the player (for continuing later where the user left off)
validation of content against full library semantics (currently only text is validated)
logging & statistics generation: e.g. use of libraries by author, view of embedded content etc. (see h5p-php-library:h5p-event-base.class.php for a list of events)
provide embed option for content (as div and as iframe)
cache assets (aggregates all css and js files into two big files to decrease http requests; done in h5p-php-library:h5p-default-storage.class.php->cacheAssets(...))
logging and statistics (there is a debug logger, but not one that allows you to log domain events)