Email Specs
Email Client Support Reference
This is the shortlist of things that actually break, rather than a full compatibility matrix. Outlook on Windows is the constraint on almost every project, because it renders with a word processing engine rather than a browser engine. Build for that and the rest tends to follow.
| Client | Engine | Practical consequence |
|---|---|---|
| Outlook on Windows | Microsoft Word based | No flexbox, no grid, limited CSS |
| Outlook on the web | Browser based | Much better support |
| Gmail web and mobile | Browser based, styles rewritten | Strips some head styles |
| Apple Mail and iOS Mail | WebKit | Best support of the major clients |
| Yahoo Mail | Browser based | Generally good, some CSS quirks |
| Technique | Safe to use | Notes |
|---|---|---|
| Table based layout | Yes | Still the reliable foundation |
| Inline CSS | Yes | Required for Gmail reliability |
| Flexbox and grid | No | Unsupported in Outlook on Windows |
| Media queries | Mostly | Ignored by some clients, design mobile first |
| Web fonts | Partially | Always provide a system font fallback |
| Background images | With care | Needs VML fallback for Outlook |
| Dark mode colour inversion | Varies | Test, some clients force invert |
Notes
Design for the lowest common denominator and enhance upward. A single column table layout with inline styles renders correctly everywhere, and progressive touches such as rounded buttons or hover states can be added knowing that losing them costs nothing. The reverse approach produces emails that only look right in the designer's own client.
Dark mode is now the most common source of rendering complaints. Some clients invert colours automatically, which turns a dark logo on a white background into an unreadable mess. Use transparent PNGs with sufficient contrast in both directions and test in Apple Mail and Outlook with dark mode enabled.
Web fonts fail silently. When they fail the fallback stack is what your subscriber reads, so choose a fallback that keeps line lengths and hierarchy close to the intended design rather than defaulting to whatever the builder inserts.
Client support shifts with every major Outlook, Gmail and Apple Mail release, and vendors rarely announce rendering changes. Check caniemail.com and run a real client test before shipping any template change.
Frequently asked questions
Do I still need table based layouts?
Yes, as long as Outlook on Windows is in your audience. Modern layout techniques are not supported there, and for most B2B lists Outlook is a large enough share that it decides the build approach.
How much testing is enough?
Apple Mail, Gmail web, Gmail mobile, Outlook on Windows and one dark mode variant covers the overwhelming majority of real rendering problems. Beyond that the returns drop off quickly.
Official sources
Platforms change specifications without notice. Check the primary documentation before a launch that depends on an exact value.
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