Classic Outlook still renders email with Word

Classic Outlook for Windows does not use a web browser to display email. It uses the rendering engine from Microsoft Word. Microsoft is moving people to the new Outlook, which uses a browser engine, but the classic app is still widely used in the corporate inboxes that matter most for business travel and events.

Word was built to lay out documents, not web pages. So:

• Flexbox and CSS grid are ignored, and layouts collapse.
• Border-radius is ignored, so rounded buttons become rectangles.
• Background images do not show without VML, an old Microsoft vector format.
• Widths need to be set as attributes, not just CSS, or content stretches to the window.
• Line heights and padding behave differently from every other client.

The unglamorous fix

Tables for layout. Fixed widths wrapped in conditional comments that only Outlook reads. VML for buttons and background images. Explicit line-height rules for Microsoft Office. None of it is visible to the reader, and all of it is the difference between an email that looks designed and one that looks broken.

It is also why a component library matters. Solve each of these once, inside the component, and nobody on the marketing team ever has to think about Word again.