CSS Properties

tab-size

CSS property · 7 supported, 0 partial, 10 unsupported across 21 clients

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

2/7 webmail, 2/6 desktop, 3/8 mobile clients support tab-size fully.
Unsupported in Gmail, Gmail Android, Gmail iOS, Outlook (New) and 6 more.
Outlook Classic (Word engine, supported to at least 2029) does not fully support tab-size, 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 Jul 21, 2022, synced by Emailens Sep 2, 2026.

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	Supported^{See caveat 1}
Outlook (New)	desktop	Outlook Web	Unsupported
Outlook Classic	desktop	Microsoft Word	Unsupported
Outlook iOS	mobile	Outlook Mobile	Supported
Outlook Android	mobile	Outlook Mobile	Unsupported
Outlook for Mac	desktop	WebKit	Supported
Apple Mail	desktop	WebKit	Supported
Apple Mail iOS	mobile	WebKit	Supported
Yahoo Mail	webmail	Yahoo	Unsupported
Yahoo Mail Android	mobile	Yahoo	Unsupported
Yahoo Mail iOS	mobile	Yahoo	Unsupported
Samsung Mail	mobile	Samsung	Supported
Thunderbird	desktop	Gecko	Unknown
HEY Mail	webmail	WebKit	Unknown
Proton Mail	webmail	Proton	Supported
AOL Mail	webmail	AOL	Unsupported
Fastmail	webmail	Fastmail	Unknown
Superhuman	desktop	Blink	Unknown

Documented caveats (1)

1.Outlook 365: Supports tab-size but strips the tab character

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

Client-by-client behaviour for tab-size

Fully supports tab-size (7): Outlook 365, Outlook iOS, Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Proton Mail.

No support (10): Gmail, Gmail Android, Gmail iOS, Outlook (New), Outlook Classic, Outlook Android, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail. Plan fallbacks for these clients before relying on tab-size in production sends.

Behaviour unverified in: Thunderbird, HEY Mail, Fastmail, Superhuman.