I'm successfully hiding target content on Outlook.com, Office 365 for Windows, and Outlook 2016 for Windows with the following code:

<style>
  [id="x_hide-outlook"]{
    display:none !important;
  }
</style>

<!--[if (gte mso 9)|(IE)]>
    <style type="text/css">
      #hide-outlook {
        display: none;
      }
    </style>
<![endif]-->

<div id="hide-outlook"><h1>HIDDEN IN OUTLOOK/OFFICE 365</h1></div>

But the above code works for everything but Office 365 / Outlook 2016 on macOS specifically (works on Windows versions).

Has anyone run into a macOS specific issue with the above logic / any solutions?