What are the core email design best practices for maximum email client compatibility?

Width: Use a max-width of 600px — most email clients, especially Outlook on Windows, render best at this width. Tables are still recommended for layout to handle Outlook's Word-based renderer.
CSS: Inline CSS is the most widely supported method. Embedded styles (<style> in <head>) are stripped by some clients (older Gmail, some mobile). External CSS is not supported in email.
Fonts: Use web-safe font stacks (Arial, Georgia, Times New Roman) with web font declarations (@font-face or Google Fonts) first, falling back gracefully. Many Outlook versions do not support web fonts.
Preheader text: Keep it 85–100 characters to display well across clients; hide any filler text using display:none or mso-hide:all.
Images: Always provide descriptive alt text. Many clients block images by default — the email must be readable without images.
Plain text: Always include a plain text version; some ESPs and spam filters penalise HTML-only emails.