outline
CSS property · 19 supported, 0 partial, 2 unsupported across 21 clients
outline is safe in 19 of 21 email clients as of Sep 2, 2026, the holdouts are Outlook (New), Outlook Classic.
- 7/7 webmail, 4/6 desktop, 8/8 mobile clients support outline fully.
- Unsupported in Outlook (New), Outlook Classic.
- Outlook Classic (Word engine, supported to at least 2029) does not fully support outline, 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 Mar 17, 2022, synced by Emailens Sep 2, 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 | Unsupported |
| Outlook Classic | desktop | Microsoft Word | Unsupported |
| 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 | Supported |
| Yahoo Mail iOS | mobile | Yahoo | Supported |
| Samsung Mail | mobile | Samsung | Supported |
| Thunderbird | desktop | Gecko | Supported |
| HEY Mail | webmail | WebKit | Supported |
| Proton Mail | webmail | Proton | Supported |
| AOL Mail | webmail | AOL | Supported |
| Fastmail | webmail | Fastmail | Supported |
| Superhuman | desktop | Blink | Supported |
Client-by-client behaviour for outline
Fully supports outline (19): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Proton Mail, AOL Mail, Fastmail, Superhuman.
No support (2): Outlook (New), Outlook Classic. Plan fallbacks for these clients before relying on outline in production sends.
Fixes & Workarounds
Outlook (New)
Unsupported"outline" is not supported in this email client.
Outlook Classic
Unsupported"outline" is not supported in this email client.
Related Features
Check your own HTML for outline
Paste your own, or run the sample below. Analyzed in this page, with no signup and nothing stored.
Test your outline layout across 21 email clients
Preview rendering, catch CSS issues, and see how outline 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 Mar 17, 2022 as the last time outline was tested in a real client, 4 years before this sync. Syncing copies that result forward, it does not re-test it. Carried in @emailens/engine data v0.12.2.