HTML & Email Structure

doctype

CSS property · 13 supported, 0 partial, 7 unsupported across 21 clients

13 of 21 email clients render doctype fully (Oct 14, 2026 data); it breaks outright in Outlook Classic, Yahoo Mail Android, Samsung Mail and 4 more.

4/7 webmail, 3/6 desktop, 6/8 mobile clients support doctype fully.
Unsupported in Outlook Classic, Yahoo Mail Android, Samsung Mail, Thunderbird and 3 more.
Outlook Classic (Word engine, supported to at least 2029) does not fully support doctype, one of the few features its replacement actually fixes; 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 25, 2019, synced by Emailens Oct 14, 2026.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Supported
Gmail Android	mobile	Gmail Mobile	Supported
Gmail iOS	mobile	Gmail Mobile	Supported
Outlook 365	webmail	Outlook Web	Supported
Outlook (New)	desktop	Outlook Web	Supported
Outlook Classic	desktop	Microsoft Word	Unsupported^{See caveat 1}
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	Unsupported
Yahoo Mail iOS	mobile	Yahoo	Supported
Samsung Mail	mobile	Samsung	Unsupported
Thunderbird	desktop	Gecko	Unsupported^{See caveat 2}
HEY Mail	webmail	WebKit	Unsupported
Proton Mail	webmail	Proton	Unsupported
AOL Mail	webmail	AOL	Unsupported
Fastmail	webmail	Fastmail	Supported
Superhuman	desktop	Blink	Unknown

Documented caveats (2)

1.Outlook Classic: Not supported. The HTML5 doctype has no impact here.
2.Thunderbird: Buggy. Renders as if there was no doctype with an Outlook email, and in HTML5 otherwise.

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

Client-by-client behaviour for doctype

Fully supports doctype (13): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook (New), Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Yahoo Mail, Yahoo Mail iOS, Fastmail.

No support (7): Outlook Classic, Yahoo Mail Android, Samsung Mail, Thunderbird, HEY Mail, Proton Mail, AOL Mail. Plan fallbacks for these clients before relying on doctype in production sends.

Behaviour unverified in: Superhuman.