CSS Properties

!important

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

!important splits the field: 10 full, 10 partial of 21 clients as of Sep 2, 2026, the partials (Gmail, Gmail Android, Gmail iOS) are where sends go wrong.

3/7 webmail, 3/6 desktop, 4/8 mobile clients support !important fully.
Partial in Gmail, Gmail Android, Gmail iOS, Outlook (New) and 6 more, behavior differs from browsers, test before relying on it.
Outlook Classic (Word engine, supported to at least 2029) does not fully support !important, 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 Nov 30, 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 2}
Gmail iOS	mobile	Gmail Mobile	Partial^{See caveat 2}
Outlook 365	webmail	Outlook Web	Supported
Outlook (New)	desktop	Outlook Web	Partial^{See caveat 3}
Outlook Classic	desktop	Microsoft Word	Partial^{See caveat 4}
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	Partial^{See caveat 5}
Yahoo Mail Android	mobile	Yahoo	Partial^{See caveat 5}
Yahoo Mail iOS	mobile	Yahoo	Partial^{See caveat 5}
Samsung Mail	mobile	Samsung	Supported
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Supported
Proton Mail	webmail	Proton	Partial^{See caveat 6}
AOL Mail	webmail	AOL	Partial^{See caveat 5}
Fastmail	webmail	Fastmail	Supported
Superhuman	desktop	Blink	Unknown

Documented caveats (6)

1.Gmail: Partial. Only supported in lowercase. (See email-bugs#13)
2.Gmail Android, Gmail iOS: Partial. Only supported in lowercase. (See email-bugs#13) Partial. Only supported inline when using a Non Gmail Account due to the lack of <style> support.
3.Outlook (New): Partial. Only supported with a space before. Partial. Not supported inline. (See email-bugs#31)
4.Outlook Classic: Partial. Only supported with a space before.
5.Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail: Partial. Removed if there is no space before when used with a background-image property. (See email-bugs#16)
6.Proton Mail: Partial. Only supported in lowercase. Partial. Only supported if not written in lowercase. Partial. Only supported inline due to the lack of <style> support.

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

Client-by-client behaviour for !important

Fully supports !important (10): Outlook 365, Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Fastmail.

Partial support (10): Gmail, Gmail Android, Gmail iOS, Outlook (New), Outlook Classic, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, Proton Mail, AOL Mail. Expect rendering quirks unique to each engine. Partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

Behaviour unverified in: Superhuman.