For CKEditor 4 CoreMedia RichText adaptions did not filter for invalid
attribute values, but just for required or forbidden attributes. This
is the behavior the mode LEGACY simulates.
LOOSE will only check, what the scheme will detect. Given the example
about numbers for STRICT mode, LOOSE will cause to accept any
character data.
Available since v11 of ckeditor-plugins and corresponding data-processing.
v10-compatibility will assume LEGACY instead.
STRICT enforces completely valid CoreMedia RichText 1.0. In addition to
LOOSE it will check for meant to be, such as a type called
Number which states to be numbers only, but regarding the schema allows
any (unchecked) character data. In case of STRICT non-numbers will be
rated invalid.
Strictness for Schema validation.