CSS Properties

system-ui

CSS property · 11 supported, 7 partial, 2 unsupported across 21 clients

11 of 21 email clients render system-ui fully (Sep 2, 2026 data); it breaks outright in Outlook (New), Outlook Classic.

5/7 webmail, 2/6 desktop, 4/8 mobile clients support system-ui fully.
Unsupported in Outlook (New), Outlook Classic.
Partial in Gmail Android, Outlook Android, Yahoo Mail Android, Samsung Mail 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 system-ui, 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 Dec 30, 2022, synced by Emailens Sep 2, 2026.

Client Support

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

Documented caveats (2)

1.Gmail, Outlook 365, Yahoo Mail, HEY Mail, Fastmail: Partial. Depends on browser support. system-ui works in all browsers. ui-serif, ui-sans-serif, ui-rounded and ui-monospace are supported on all browsers on iOS and Safari on MacOS.
2.Gmail Android, Outlook Android, Yahoo Mail Android, Samsung Mail, Thunderbird, Proton Mail, AOL Mail: Partial. Supports system-ui only.

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

Client-by-client behaviour for system-ui

Fully supports system-ui (11): Gmail, Gmail iOS, Outlook 365, Outlook iOS, Outlook for Mac, Apple Mail, Apple Mail iOS, Yahoo Mail, Yahoo Mail iOS, HEY Mail, Fastmail.

Partial support (7): Gmail Android, Outlook Android, Yahoo Mail Android, Samsung Mail, Thunderbird, Proton 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.

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

Behaviour unverified in: Superhuman.