A formatting issue may be seen in the Portal on an Interactive Documentation page which was generated by a WADL. No errors are seen in the logs as it is just a formatting/display issue.
This impacts all API Portal versions 3.5 (including CRs) and older.
In the /SYSTEM/conf/properties.xml file, the property named enableHtml5InEditors may have a value of "no" instead of "yes". A value of "yes" is default, so if the value shows "no" then it was customized.
To resolve, the value of the enableHtml5InEditors attribute in the /SYSTEM/conf/properties.xml file will need to be set to a value of "yes" so that it should read as follows:
<Property name="enableHtml5InEditors" value="yes" />