An attribute containing an unconstrained string which may contain any sequence of UTF-8 encoded characters.
The standard rules for escaping XML special characters apply, i.e. escape
' with ' (if single quotes are used to quote the value)
" with " (if single quotes are used to quote the value)
< with <
& with &
Other XML named entity sequences such as > MAY be used and MUST be interpreted as such by recipients if present, encoded as a JSON string.
Instances of the double quote character MUST be escaped with a single backslash.