Pseudo-Classes & Selectors

:universal-selector

selector or pseudo-class · 15 supported, 2 partial, 3 unsupported across 21 clients

The universal selector (*) matches every element. It is occasionally used in email for a single global rule like setting box-sizing: border-box, but its specificity is extremely low and Outlook ignores it because <style> blocks are stripped. In practice, the universal selector is a niche progressive enhancement.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Supported
Gmail Android	mobile	Gmail Mobile	Partial
Gmail iOS	mobile	Gmail Mobile	Partial
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	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

Client-by-client behaviour for :universal-selector

Fully supports :universal-selector (15): Gmail, Outlook 365, Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, Samsung Mail, Thunderbird, HEY Mail, AOL Mail, Fastmail.

Partial support (2): Gmail Android, Gmail 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 (3): Outlook (New), Outlook Classic, Proton Mail. Plan fallbacks for these clients before relying on :universal-selector in production sends.

Behaviour unverified in: Superhuman.

When to use :universal-selector in email

Setting box-sizing: border-box across all elements in modern clients.
Resetting margin and padding globally as the start of a custom design system.
Applying a single dark-mode color override to every element via *.

Rendering behaviour and edge cases

Specificity is the lowest possible , any other selector overrides *.
Outlook on Windows ignores * because <style> is stripped.
Universal selectors can have measurable rendering cost on slow clients with large emails.

Recommended fallback strategy

Avoid relying on * for production emails. If you need a global reset, apply explicit inline styles on each element instead. Reserve the universal selector for niche progressive-enhancement cases in modern clients only.