Allow comments needed for VML
Because the Outlook client uses Word as rendering the use of HTML is very limited. To work around this comments like this "<!--[if gte mso 9]>" is used to insert VML when rendered in a client.
However these tags are stripped away when using item.body.setSelectedDataAsync to insert HTML from an add-in.
Please allow these comments tags to control VML to be passed through.
Thomas Sørensen commented
The issue is also described in this Stackoverflow question: https://stackoverflow.com/questions/48281418/outlook-add-in-insert-html-with-background-image