HTML Elements

<base>

HTML element · 5 supported, 12 partial, 3 unsupported across 21 clients

As of Sep 2, 2026, <base> fails or degrades in 16 of 21 email clients, treat it as progressive enhancement, never structure.

0/7 webmail, 3/6 desktop, 2/8 mobile clients support <base> fully.
Unsupported in Outlook Classic, Outlook Android, Proton Mail.
Partial in Gmail, Gmail Android, Gmail iOS, Outlook 365 and 8 more, behavior differs from browsers, test before relying on it.
Outlook Classic (Word engine, supported to at least 2029) does not fully support <base>, 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 Dec 1, 2021, synced by Emailens Sep 2, 2026.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Partial^{See caveat 1}
Gmail Android	mobile	Gmail Mobile	Partial^{See caveat 1}
Gmail iOS	mobile	Gmail Mobile	Partial^{See caveat 1}
Outlook 365	webmail	Outlook Web	Partial^{See caveat 1}
Outlook (New)	desktop	Outlook Web	Supported
Outlook Classic	desktop	Microsoft Word	Unsupported
Outlook iOS	mobile	Outlook Mobile	Partial^{See caveat 2}
Outlook Android	mobile	Outlook Mobile	Unsupported
Outlook for Mac	desktop	WebKit	Partial^{See caveat 1}
Apple Mail	desktop	WebKit	Supported
Apple Mail iOS	mobile	WebKit	Supported
Yahoo Mail	webmail	Yahoo	Partial^{See caveat 3}
Yahoo Mail Android	mobile	Yahoo	Partial^{See caveat 3}
Yahoo Mail iOS	mobile	Yahoo	Partial^{See caveat 3}
Samsung Mail	mobile	Samsung	Supported
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Partial^{See caveat 1}
Proton Mail	webmail	Proton	Unsupported
AOL Mail	webmail	AOL	Partial^{See caveat 3}
Fastmail	webmail	Fastmail	Partial^{See caveat 4}
Superhuman	desktop	Blink	Unknown

Documented caveats (4)

1.Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook for Mac, HEY Mail: Buggy. The element is removed but URLs are turned into links with href attribute.
2.Outlook iOS: Buggy. The element is removed but URLs are turned into links with href attribute. Buggy. URLs are computed based on the first <base> element found in the <body>.
3.Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail: Buggy. The element is removed but URLs are turned into links with href attribute. Buggy. URLs are computed based on the last <base> element found in the <body>. Buggy. Not supported for local anchor links.
4.Fastmail: Buggy. The element is removed but URLs are turned into links with href attribute. Buggy. Not supported for local anchor links.

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

Client-by-client behaviour for <base>

Fully supports <base> (5): Outlook (New), Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird.

Partial support (12): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook iOS, Outlook for Mac, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, HEY Mail, AOL Mail, Fastmail. 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 Classic, Outlook Android, Proton Mail. Plan fallbacks for these clients before relying on <base> in production sends.

Behaviour unverified in: Superhuman.