Pseudo-Classes & Selectors

:chaining

selector or pseudo-class · 14 supported, 2 partial, 4 unsupported across 21 clients

Selector chaining (combining classes or tags without a separator, e.g., .button.primary) increases specificity by requiring multiple conditions on the same element. In email, chained selectors live in <style> blocks and only apply where <style> is preserved.

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	Unsupported
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 :chaining

Fully supports :chaining (14): Gmail, 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 (4): Outlook 365, Outlook (New), Outlook Classic, Proton Mail. Plan fallbacks for these clients before relying on :chaining in production sends.

Behaviour unverified in: Superhuman.

When to use :chaining in email

Targeting a class only when combined with another modifier class.
Increasing specificity to override a default rule without using !important.
Constraining styles to elements with both a tag and class match.

Rendering behaviour and edge cases

Outlook strips <style>, so chained selectors do not apply there.
Gmail's class-rewriting can break selectors that depend on multiple class names matching.
Specificity from chaining is additive , a chained selector outranks a single class.

Recommended fallback strategy

Use chained selectors in <style> blocks for progressive enhancement only. For Outlook compatibility, inline the equivalent styles on the matching elements directly.