<dfn>
HTML element · 16 supported, 2 partial, 0 unsupported across 21 clients
<dfn> splits the field: 16 full, 2 partial of 21 clients as of Sep 2, 2026, the partials (Outlook (New), Outlook Classic) are where sends go wrong.
- 5/7 webmail, 3/6 desktop, 8/8 mobile clients support <dfn> fully.
- Partial in Outlook (New), Outlook Classic, behavior differs from browsers, test before relying on it.
- Outlook Classic (Word engine, supported to at least 2029) does not fully support <dfn>, 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 Sep 11, 2023, 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 | PartialSee caveat 1 |
| Outlook Classic | desktop | Microsoft Word | PartialSee 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 | Supported |
| Yahoo Mail iOS | mobile | Yahoo | Supported |
| Samsung Mail | mobile | Samsung | Supported |
| Thunderbird | desktop | Gecko | Supported |
| HEY Mail | webmail | WebKit | Unknown |
| Proton Mail | webmail | Proton | Supported |
| AOL Mail | webmail | AOL | Supported |
| Fastmail | webmail | Fastmail | Unknown |
| Superhuman | desktop | Blink | Unknown |
Documented caveats (1)
- 1.Outlook (New), Outlook Classic: Partial. The
titleattribute is removed.
Caveats recorded by caniemail.com (MIT) and carried verbatim, one entry per wording rather than per client.
Client-by-client behaviour for <dfn>
Fully supports <dfn> (16): 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, Proton Mail, AOL Mail.
Partial support (2): Outlook (New), Outlook Classic. 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: HEY Mail, Fastmail, Superhuman.
Fixes & Workarounds
Outlook (New)
Partial`<dfn>` is not supported in this email client.
Outlook Classic
Partial`<dfn>` is not supported in this email client.
Related Features
Check your own HTML for <dfn>
Paste your own, or run the sample below. Analyzed in this page, with no signup and nothing stored.
Test your <dfn> layout across 21 email clients
Preview rendering, catch CSS issues, and see how <dfn> 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 Sep 11, 2023 as the last time <dfn> was tested in a real client, 2 years before this sync. Syncing copies that result forward, it does not re-test it. Carried in @emailens/engine data v0.12.2.