This is a simplified version of the CKEDITOR.filter.allowedContentRules type.
It may contain only one element and its styles, classes, and attributes. Only the
string format and a CKEDITOR.style instances are accepted. Required properties
are not allowed in this format.
'img[src,alt](foo)' // Correct rule.
'ol, ul(!foo)' // Incorrect rule. Multiple elements and required
// properties are not supported.