custom-properties
CSS property · 7 supported, 0 partial, 13 unsupported across 21 clients
As of Sep 2, 2026, custom-properties fails or degrades in 14 of 21 email clients, treat it as progressive enhancement, never structure.
- 2/7 webmail, 3/6 desktop, 2/8 mobile clients support custom-properties fully.
- Unsupported in Gmail, Gmail Android, Gmail iOS, Outlook 365 and 9 more.
- Outlook Classic (Word engine, supported to at least 2029) does not fully support custom-properties, 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 Feb 25, 2020, synced by Emailens Sep 2, 2026.
Client Support
| Client | Category | Engine | Support |
|---|---|---|---|
| Gmail | webmail | Gmail Web | UnsupportedSee caveat 1 |
| Gmail Android | mobile | Gmail Mobile | UnsupportedSee caveat 2 |
| Gmail iOS | mobile | Gmail Mobile | UnsupportedSee caveat 2 |
| Outlook 365 | webmail | Outlook Web | Unsupported |
| Outlook (New) | desktop | Outlook Web | Unsupported |
| Outlook Classic | desktop | Microsoft Word | Unsupported |
| Outlook iOS | mobile | Outlook Mobile | Unsupported |
| 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 | Supported |
| HEY Mail | webmail | WebKit | Supported |
| Proton Mail | webmail | Proton | Supported |
| AOL Mail | webmail | AOL | Unsupported |
| Fastmail | webmail | Fastmail | UnsupportedSee caveat 1 |
| Superhuman | desktop | Blink | Unknown |
Documented caveats (2)
- 1.Gmail, Fastmail: Partial. The
var()function is supported, but not the variable declaration. - 2.Gmail Android, Gmail iOS: Partial. The
var()function is supported, but not the variable declaration. Partial. Not supported with non Google accounts.
Caveats recorded by caniemail.com (MIT) and carried verbatim, one entry per wording rather than per client.
Client-by-client behaviour for custom-properties
Fully supports custom-properties (7): Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Proton Mail.
No support (13): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook (New), Outlook Classic, Outlook iOS, Outlook Android, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail, Fastmail. Plan fallbacks for these clients before relying on custom-properties in production sends.
Behaviour unverified in: Superhuman.
Fixes & Workarounds
Gmail
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Gmail Android
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Gmail iOS
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Outlook 365
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Outlook (New)
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Outlook Classic
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Outlook iOS
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Outlook Android
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Yahoo Mail
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Yahoo Mail Android
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Yahoo Mail iOS
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
AOL Mail
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Fastmail
UnsupportedInline CSS custom properties (variables) into static literal values; Gmail, Outlook, and Yahoo do not support CSS variables.
Related Features
Check your own HTML for custom-properties
Paste your own, or run the sample below. Analyzed in this page, with no signup and nothing stored.
Test your custom-properties layout across 21 email clients
Preview rendering, catch CSS issues, and see how custom-properties displays across Outlook, Gmail, and Apple Mail. Free, no account needed.
Instant live preview · No signup required · 21 email engines
Last tested upstream: · Last synced by Emailens:
caniemail.com (MIT) records Feb 25, 2020 as the last time custom-properties was tested in a real client, 6 years before this sync. Syncing copies that result forward, it does not re-test it. Carried in @emailens/engine data v0.12.2.