Hello,
Ok, Got it, I don’t know why suitecrm haven’t added code to parse html into Detailview.
You can achieve it, go to custom/modules/Cases/metadata/detailviewdefs.php, if it doesn’t exist copy modules/Cases/metadata/detailviewdefs.php to custom/modules/Cases/metadata/detailviewdefs.php and find description field and write code as below.
0 =>
array(
'name' => 'description',
'customCode' => '{$fields.description.value|escape:\'htmlentitydecode\' |escape:\'html\'|strip_tags|url2html|nl2br}',
), Do quick repair & rebuild.
You are done!