CSS Properties

font-size-adjust

CSS property · 9 supported, 5 partial, 2 unsupported across 21 clients

9 of 21 email clients render font-size-adjust fully (Sep 2, 2026 data); it breaks outright in Outlook (New), Outlook Classic.

3/7 webmail, 3/6 desktop, 3/8 mobile clients support font-size-adjust fully.
Unsupported in Outlook (New), Outlook Classic.
Partial in Gmail Android, Gmail iOS, Yahoo Mail, Yahoo Mail Android and 1 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-adjust, 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 Apr 16, 2026, synced by Emailens Sep 2, 2026.

Client Support

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

Documented caveats (2)

1.Gmail Android, Gmail iOS: Not supported with non Google accounts.
2.Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS: 2-value syntax is supported, but the first value (font-metric) is ignored.

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

Client-by-client behaviour for font-size-adjust

Fully supports font-size-adjust (9): Gmail, Outlook 365, Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Thunderbird, Proton Mail.

Partial support (5): Gmail Android, Gmail iOS, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS. Expect rendering quirks unique to each engine. Partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

No support (2): Outlook (New), Outlook Classic. Plan fallbacks for these clients before relying on font-size-adjust in production sends.

Behaviour unverified in: Samsung Mail, HEY Mail, AOL Mail, Fastmail, Superhuman.