Please confirm the "Read all standard mail in plain text" option is unselected via File>Options>Trust Center>Trust Center Settings…> Email Security>Read as Plain Text. When the below option is set as Plain Text, the new emails you create will be displayed as plain text.
How do I view an HTML email in Outlook 2016?
Change the message format for one message With the message open, on the Message Options tab, in the Format group, click the format that you want to use—Plain Text, HTML, or Rich Text.
How do I import HTML into Outlook 2016?
Stick to tables Most Outlook versions on Windows don't have good support for the CSS box model or things like flexbox, CSS Grid, and floats. This makes it hard to use semantic HTML to build email layouts that display properly in Outlook.