The handling of usernames should be case-insensitive in core MediaWiki.
This means, possibly among other caveats:
- 'QWERTY' and 'qwerty' and 'Qwerty' should be considered the same username for the purposes of account creation and logging in
- Titles in the User: namespace should be case-insensitive like in the Special: namespace (related to bug T2453)
- Usernames starting with a lowercase first letter should be permitted (bug T3574 / bug T28396), and preferably users should be able to change the casing on already existing accounts themselves