Pseudo-Classes & Selectors

:attribute-selector

selector or pseudo-class · 7 supported, 10 partial, 3 unsupported across 21 clients

As of Sep 2, 2026, :attribute-selector fails or degrades in 14 of 21 email clients, treat it as progressive enhancement, never structure.

2/7 webmail, 3/6 desktop, 2/8 mobile clients support :attribute-selector fully.
Unsupported in Outlook (New), Outlook Classic, Proton Mail.
Partial in Gmail, Gmail Android, Gmail iOS, Outlook 365 and 6 more, behavior differs from browsers, test before relying on it.
Outlook Classic (Word engine, supported to at least 2029) does not fully support :attribute-selector, 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 15, 2020, synced by Emailens Sep 2, 2026.

Attribute selectors target elements based on their attributes (e.g., [type=email], [data-id="123"]). Email support is limited because many clients strip <style> blocks where these selectors live. When <style> blocks are preserved, attribute selectors work in modern clients but are ignored in Outlook on Windows.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Partial^{See caveat 1}
Gmail Android	mobile	Gmail Mobile	Partial^{See caveat 2}
Gmail iOS	mobile	Gmail Mobile	Partial^{See caveat 2}
Outlook 365	webmail	Outlook Web	Partial^{See caveat 3}
Outlook (New)	desktop	Outlook Web	Unsupported
Outlook Classic	desktop	Microsoft Word	Unsupported
Outlook iOS	mobile	Outlook Mobile	Partial^{See caveat 4}
Outlook Android	mobile	Outlook Mobile	Partial^{See caveat 4}
Outlook for Mac	desktop	WebKit	Supported
Apple Mail	desktop	WebKit	Supported
Apple Mail iOS	mobile	WebKit	Supported
Yahoo Mail	webmail	Yahoo	Partial^{See caveat 5}
Yahoo Mail Android	mobile	Yahoo	Partial^{See caveat 5}
Yahoo Mail iOS	mobile	Yahoo	Partial^{See caveat 5}
Samsung Mail	mobile	Samsung	Supported
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Supported
Proton Mail	webmail	Proton	Unsupported
AOL Mail	webmail	AOL	Partial^{See caveat 5}
Fastmail	webmail	Fastmail	Supported
Superhuman	desktop	Blink	Unknown

Documented caveats (5)

1.Gmail: Partial. Only supports [attr~=value] syntax. Only class as an attribute name is known to be supported.
2.Gmail Android, Gmail iOS: Partial. Only supports [attr~=value] syntax. Only class as an attribute name is known to be supported. Partial. Doesn't work with non Google accounts.
3.Outlook 365: Partial. Can not be used with a class selector (.test[class]). Must be used solo ([class]) or with an element selector (td[class]). Partial. Only supports [attr], [attr=value], [attr~=value], [attr|=value] syntaxes. Buggy. A class="test" in the HTML is prefixed class="x_test", but an attribute selector stays unprefixed [class="test"].
4.Outlook iOS, Outlook Android: Partial. Can not be used with a class selector (.test[class]). Must be used solo ([class]) or with an element selector (td[class]). Partial. Only supports [attr], [attr=value], [attr~=value], [attr|=value] syntaxes.
5.Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail: Partial. Only supports [attr=value] syntax.

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

Client-by-client behaviour for :attribute-selector

Fully supports :attribute-selector (7): Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Fastmail.

Partial support (10): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook iOS, Outlook Android, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, 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 (3): Outlook (New), Outlook Classic, Proton Mail. Plan fallbacks for these clients before relying on :attribute-selector in production sends.

Behaviour unverified in: Superhuman.

When to use :attribute-selector in email

Targeting form inputs by type in AMP for Email contexts.
Selecting elements by data-* attributes for dark-mode style overrides.
Cross-cutting styles based on aria-* attributes for accessibility consistency.

Rendering behaviour and edge cases

Outlook strips <style> blocks, neutralizing attribute selectors.
Gmail rewrites class names but preserves attribute selectors when <style> blocks survive.
Apple Mail honors attribute selectors reliably across recent macOS and iOS releases.

Recommended fallback strategy

Use attribute selectors only when <style> blocks are preserved. For Outlook coverage, duplicate the styling inline on the matching elements. Treat attribute-based selectors as progressive enhancement, never as the only path to a critical style.