Definition
Client support describes the varying levels of HTML and CSS feature support across different email clients (Gmail, Outlook, Apple Mail, Yahoo, etc.). Unlike web browsers, which largely follow web standards, email clients each apply their own rendering engine and CSS transformations, creating significant inconsistencies in how emails display.
Understanding client support is essential for email designers to create emails that work reliably across the diverse ecosystem of email clients their subscribers use.
Client Support Tiers
| Tier | Description | Examples |
|---|---|---|
| Full | Broad CSS support, modern standards | Apple Mail, Outlook for iOS/Android |
| Moderate | Good support with known gaps | Outlook.com, Yahoo Mail, Samsung Mail |
| Limited | Restricted CSS, basic rendering | Gmail (significant CSS stripping) |
| Legacy | Outlook-specific rendering engine | Outlook for Windows (Word rendering) |
Key Feature Support
| Feature | Apple Mail | Gmail | Outlook.com | Outlook (Win) | Yahoo |
|---|---|---|---|---|---|
| Media queries | Yes | Yes | Yes | No | Yes |
| Web fonts | Yes | No | Limited | No | Limited |
| CSS animations | Yes | No | Some | No | No |
| Background images | Yes | No | Yes | No | Limited |
| Border-radius | Yes | Yes | Yes | No | Yes |
| Flexbox | Yes | Partial | Partial | No | Partial |
| CSS Grid | Yes | No | No | No | No |
| AMP for Email | No | Yes | No | No | No |
Testing for Client Support
- Use email testing tools: Litmus and Email on Acid provide rendering previews across 100+ clients
- Build progressively: Design for the lowest common denominator, then enhance for better-supported clients
- Check analytics: Test against the email clients your specific audience uses most
- Maintain a support matrix: Document which features work in which clients for your reference
- Use fallbacks: Always provide fallback styling for unsupported properties
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Related Glossary Terms
Above the Fold in Email
Above the fold in email refers to the content visible to a recipient before they scroll, a critical area for first impressions and primary calls to action.
Accessible CTA (Email)
An accessible CTA is a button or link coded with sufficient colour contrast, visible focus states, descriptive screen-reader text, and a minimum touch target of 44×44 pixels.
Adaptive Design
Adaptive design in email uses predefined layouts that adjust to specific device or client widths, rather than fluidly resizing to every possible screen.
AI-Generated Content in Email
AI-generated content in email is copy, images, code or subject lines produced by artificial intelligence tools to speed up campaign production and testing.
Alt Text
Alt text is the written alternative to an image in an email, displayed when images are blocked, slow to load, or consumed by screen readers.
Autoplay Video in Email
Autoplay video in email refers to the limited ability to play video content automatically when an email is opened, using specific client-supported techniques.
Frequently Asked Questions
Apple Mail offers the broadest CSS support, following web standards most closely. Outlook for iOS/Android also provides excellent support. Gmail has the most significant CSS limitations, stripping many properties during rendering.
Outlook for Windows uses Microsoft Word's rendering engine, which severely limits CSS support. Use table-based layouts, inline styles, avoid background images, and provide VML fallbacks for advanced visual elements. Test every template specifically in Outlook for Windows.
Email client support changes infrequently but does evolve. Google, Microsoft, and Apple periodically update their rendering engines. Subscribe to email design newsletters and testing tool changelogs to stay informed about support changes.