Why It Matters
Your beautifully designed email won't look the same everywhere. Outlook still uses Word's rendering engine (yes, really), which means CSS that works perfectly in Gmail can break completely in Outlook. Apple Mail handles most modern CSS well, while some older Android clients strip styles entirely. If you don't test across clients, a chunk of your audience sees a broken mess.
How It Works
Email clients connect to mail servers using protocols like IMAP, POP3, or proprietary APIs. They download or sync messages and render the HTML and CSS in the email body. The catch: there's no universal standard for email rendering. Each client has its own quirks, its own level of CSS support, and its own way of handling images, fonts, and interactive elements.
Quick Tips
- Test your emails in at least Gmail, Outlook, and Apple Mail — they cover roughly 80% of market share
- Use inline CSS rather than stylesheet blocks; Outlook and some mobile clients strip
<style>tags - Always include alt text on images — some clients block images by default, and your message should still make sense without them