Pseudo-Classes & Selectors

:adjacent-sibling

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

The adjacent sibling combinator (+) selects an element that immediately follows another at the same level. It is supported in modern Apple Mail and webmail clients when <style> blocks survive, and ignored in Outlook for the usual reason. It is most useful for spacing relationships between adjacent content blocks.

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 :adjacent-sibling

Fully supports :adjacent-sibling (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 :adjacent-sibling in production sends.

Behaviour unverified in: Superhuman.

When to use :adjacent-sibling in email

Adding extra top-margin between adjacent paragraphs in long-form content.
Styling the first heading after an image differently than other headings.
Creating visual rhythm between sequential card-style sections.

Rendering behaviour and edge cases

Outlook on Windows ignores adjacent sibling selectors because <style> is stripped.
Gmail rewrites class names, which can break adjacent-sibling selectors that rely on class matching.
Apple Mail supports the combinator reliably in recent macOS and iOS releases.

Recommended fallback strategy

Replicate the spacing relationships using explicit inline margin or padded cells for Outlook coverage. Treat adjacent-sibling styling as progressive enhancement.