font-size

Client-by-client behaviour for font-size

Fully supports font-size (10): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook iOS, Outlook Android, Apple Mail, Apple Mail iOS, Thunderbird, Superhuman.

Partial support (4): Outlook (New), Outlook Classic, Yahoo Mail, Samsung Mail. Expect rendering quirks unique to each engine — partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

Behaviour unverified in: HEY Mail.

When to use font-size in email

• Scaling headlines down on mobile via media queries when targeting clients that honor them.
• Setting consistent body copy size across nested tables to prevent inheritance drift.
• Establishing visual hierarchy between hero, secondary, and footer copy.

Rendering behaviour and edge cases

• Apple Mail on iOS auto-enlarges text below ~13px for accessibility, regardless of your declared size.
• Gmail Android sometimes scales text proportionally when the user has accessibility text-size settings enabled.
• Older Outlook builds inherit Word's font-size logic, which can compound nested em-based sizes unexpectedly.

Recommended fallback strategy

Use absolute units (px) over em or rem in email for predictable rendering. Set font-size on every text-bearing element rather than relying on inheritance, since some clients reset font-size at table boundaries.

Fixes & Workarounds