Tables, not divs
Flexbox and grid do not exist in classic Outlook. Nested tables with explicit widths are the only layout that works everywhere.
Inline styles
Gmail removes style blocks. Every style must be inline on the element.
Images
Hosted by HTTPS URL, with width and height attributes, display:block, and alt text. Never base64, never cid attachments.
Fonts
Only fonts installed on the reader's device render. Use system, Arial, Georgia, Verdana, Trebuchet. Web fonts are ignored by most clients.
Unsupported
Background images in Outlook, border-radius in classic Outlook, CSS animations everywhere, forms, scripts, video.