CSS Properties

font-size

CSS property · 12 supported, 7 partial, 0 unsupported across 21 clients

font-size splits the field: 12 full, 7 partial of 21 clients as of Sep 2, 2026, the partials (Outlook (New), Outlook Classic, Yahoo Mail) are where sends go wrong.

3/7 webmail, 4/6 desktop, 5/8 mobile clients support font-size fully.
Partial in Outlook (New), Outlook Classic, Yahoo Mail, Yahoo Mail Android and 3 more, behavior differs from browsers, test before relying on it.
Outlook Classic (Word engine, supported to at least 2029) does not fully support font-size, and its replacement changes little here; see the deprecation diff.
Across all clients, only 6 of 298 tracked HTML/CSS features render identically everywhere.

Source: caniemail.com (MIT), via the Emailens engine. Last tested upstream Feb 28, 2024, synced by Emailens Sep 2, 2026.

font-size sets the rendered size of text in email and is supported universally, but client-specific minimum-font-size settings can override your declaration. iOS Mail, for example, may upscale anything below 13–14px to improve legibility, breaking carefully tuned typographic hierarchies.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Supported
Gmail Android	mobile	Gmail Mobile	Supported
Gmail iOS	mobile	Gmail Mobile	Supported
Outlook 365	webmail	Outlook Web	Supported
Outlook (New)	desktop	Outlook Web	Partial^{See caveat 1}
Outlook Classic	desktop	Microsoft Word	Partial^{See caveat 2}
Outlook iOS	mobile	Outlook Mobile	Supported
Outlook Android	mobile	Outlook Mobile	Supported
Outlook for Mac	desktop	WebKit	Supported
Apple Mail	desktop	WebKit	Supported
Apple Mail iOS	mobile	WebKit	Supported
Yahoo Mail	webmail	Yahoo	Partial^{See caveat 1}
Yahoo Mail Android	mobile	Yahoo	Partial^{See caveat 1}
Yahoo Mail iOS	mobile	Yahoo	Partial^{See caveat 1}
Samsung Mail	mobile	Samsung	Partial^{See caveat 2}
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Unknown
Proton Mail	webmail	Proton	Supported
AOL Mail	webmail	AOL	Partial^{See caveat 1}
Fastmail	webmail	Fastmail	Unknown
Superhuman	desktop	Blink	Supported

Documented caveats (2)

1.Outlook (New), Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail: Partial support. rem values are not supported.
2.Outlook Classic, Samsung Mail: Partial support. relative and percentage size values not supported.

Caveats recorded by caniemail.com (MIT) and carried verbatim, one entry per wording rather than per client.

Client-by-client behaviour for font-size

Fully supports font-size (12): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Thunderbird, Proton Mail, Superhuman.

Partial support (7): Outlook (New), Outlook Classic, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, Samsung Mail, AOL 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, Fastmail.

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.