<h1>
HTML element · 20 supported, 0 partial, 0 unsupported across 21 clients
<h1> is supported in every email client where behavior is verified, 20 of 21 as of Sep 2, 2026; Superhuman remains untested.
- 7/7 webmail, 5/6 desktop, 8/8 mobile clients support <h1> fully.
- 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 May 8, 2020, synced by Emailens Sep 2, 2026.
The <h1> element marks the primary heading of a document and is supported by every email client. Default browser styling for <h1> is large and bold with significant top and bottom margin, which is rarely what email designs intend. Always reset <h1> styling inline to prevent client-default spacing from disrupting the layout.
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 | Supported |
| Outlook Classic | desktop | Microsoft Word | Supported |
| 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 | Unknown |
Client-by-client behaviour for <h1>
Fully supports <h1> (20): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook (New), Outlook Classic, 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.
Behaviour unverified in: Superhuman.
When to use <h1> in email
- The headline of a marketing email's hero section.
- Major section titles in long-form newsletter content.
- Article-style transactional confirmations with a clear primary message.
Rendering behaviour and edge cases
- Default font-size and margin vary widely between clients , explicitly set both inline.
- Outlook on Windows respects <h1> styling but inherits Word-style typography that may surprise web developers.
- Some clients down-rank <h1> in dark mode, applying their own contrast adjustments.
Recommended fallback strategy
Override default <h1> styling with explicit inline font-size, font-weight, line-height, and margin. Treat the inline declaration as authoritative , do not rely on client defaults for a hero headline.
Related Features
Check your own HTML for <h1>
Paste your own, or run the sample below. Analyzed in this page, with no signup and nothing stored.
Test your <h1> layout across 21 email clients
Preview rendering, catch CSS issues, and see how <h1> 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 May 8, 2020 as the last time <h1> 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.