Pseudo-Classes & Selectors

pseudo-class-default

CSS property · 10 supported, 2 partial, 5 unsupported across 21 clients

10 of 21 email clients render pseudo-class-default fully (Oct 14, 2026 data); it breaks outright in Gmail, Gmail Android, Gmail iOS and 2 more.

2/7 webmail, 3/6 desktop, 5/8 mobile clients support pseudo-class-default fully.
Unsupported in Gmail, Gmail Android, Gmail iOS, Outlook Classic and 1 more.
Partial in Outlook 365, Outlook (New), behavior differs from browsers, test before relying on it.
Outlook Classic (Word engine, supported to at least 2029) does not fully support pseudo-class-default, 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 5, 2026, synced by Emailens Oct 14, 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	Partial^{See caveat 1}
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	Supported
Yahoo Mail Android	mobile	Yahoo	Supported
Yahoo Mail iOS	mobile	Yahoo	Supported
Samsung Mail	mobile	Samsung	Unknown
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Unknown
Proton Mail	webmail	Proton	Unsupported
AOL Mail	webmail	AOL	Supported
Fastmail	webmail	Fastmail	Unknown
Superhuman	desktop	Blink	Unknown

Documented caveats (1)

1.Outlook 365, Outlook (New): Works on input[radio] and input[checkbox] only.

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

Client-by-client behaviour for pseudo-class-default

Fully supports pseudo-class-default (10): Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, Thunderbird, AOL Mail.

Partial support (2): Outlook 365, Outlook (New). Expect rendering quirks unique to each engine. Partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

No support (5): Gmail, Gmail Android, Gmail iOS, Outlook Classic, Proton Mail. Plan fallbacks for these clients before relying on pseudo-class-default in production sends.

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