Deprecation: #63974 - Deprecate CSS compressor callback method 

See forge#63974

Description 

The callback method compressCssPregCallback() as defined in EXT:core/Classes/Resource/ResourceCompressor.php has been marked as deprecated due to the overhauled regular expressions.

Impact 

Usage of the mentioned method is discouraged and will break as of CMS 8.

Affected installations 

All installations or extensions using the compressCssPregCallback() callback method.

Migration 

No migration possible for the callback method.