Pseudo-Elements

::first-line

pseudo-element · 6 supported, 2 partial, 6 unsupported across 15 clients

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
Outlook (New)	desktop	Outlook Web	Unsupported
Outlook Classic	desktop	Microsoft Word	Unsupported
Outlook iOS	mobile	Outlook Mobile	Partial
Outlook Android	mobile	Outlook Mobile	Unsupported
Apple Mail	desktop	WebKit	Supported
Apple Mail iOS	mobile	WebKit	Supported
Yahoo Mail	webmail	Yahoo	Supported
Samsung Mail	mobile	Samsung	Supported
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Supported
Superhuman	desktop	Blink	Unknown

Client-by-client behaviour for ::first-line

Fully supports ::first-line (6): Apple Mail, Apple Mail iOS, Yahoo Mail, Samsung Mail, Thunderbird, HEY Mail.

Partial support (2): Outlook 365, Outlook 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 (6): Gmail, Gmail Android, Gmail iOS, Outlook (New), Outlook Classic, Outlook Android. Plan fallbacks for these clients before relying on ::first-line in production sends.

Behaviour unverified in: Superhuman.