The MW core Sanitizer allows all properties except for accelerator, -o-link, -o-link-source and -o-replace. It allows all values except when they contain the functions url(), src(), image(), image-set(), or attr() with the url syntax type. We could do the same in TemplateStyles.
Unknown properties would still have to be syntactically valid. We could have a generic matcher that would check for bad functions or URL tokens.
CSS is complex and constantly changing. Most changes are not relevant for security. There is @supportsto help users develop CSS which works across all platforms.