Pseudo-Elements

::first-letter

pseudo-element · 10 supported, 3 partial, 7 unsupported across 21 clients

10 of 21 email clients render ::first-letter fully (Sep 2, 2026 data); it breaks outright in Gmail, Gmail Android, Gmail iOS and 4 more.

4/7 webmail, 2/6 desktop, 4/8 mobile clients support ::first-letter fully.
Unsupported in Gmail, Gmail Android, Gmail iOS, Outlook (New) and 3 more.
Partial in Outlook 365, Outlook iOS, Outlook for Mac, behavior differs from browsers, test before relying on it.
Outlook Classic (Word engine, supported to at least 2029) does not fully support ::first-letter, 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 May 21, 2020, synced by Emailens Sep 2, 2026.

Client Support

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

Documented caveats (2)

1.Outlook 365, Outlook iOS: Partial. Only supported on type selectors.
2.Outlook for Mac: Not supported with non Outlook accounts.

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

Client-by-client behaviour for ::first-letter

Fully supports ::first-letter (10): Apple Mail, Apple Mail iOS, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, Samsung Mail, Thunderbird, HEY Mail, AOL Mail, Fastmail.

Partial support (3): Outlook 365, Outlook iOS, Outlook for Mac. Expect rendering quirks unique to each engine. Partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

No support (7): Gmail, Gmail Android, Gmail iOS, Outlook (New), Outlook Classic, Outlook Android, Proton Mail. Plan fallbacks for these clients before relying on ::first-letter in production sends.

Behaviour unverified in: Superhuman.