CSS Properties

text-decoration-thickness

CSS property · 8 supported, 0 partial, 12 unsupported across 21 clients

8 of 21 email clients render text-decoration-thickness fully (Sep 2, 2026 data); it breaks outright in Gmail, Gmail Android, Gmail iOS and 9 more.

3/7 webmail, 3/6 desktop, 2/8 mobile clients support text-decoration-thickness fully.
Unsupported in Gmail, Gmail Android, Gmail iOS, Outlook 365 and 8 more.
Outlook Classic (Word engine, supported to at least 2029) does not fully support text-decoration-thickness, 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 Jan 16, 2023, synced by Emailens Sep 2, 2026.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Unsupported
Gmail Android	mobile	Gmail Mobile	Unsupported
Gmail iOS	mobile	Gmail Mobile	Unsupported
Outlook 365	webmail	Outlook Web	Unsupported
Outlook (New)	desktop	Outlook Web	Unsupported^{See caveat 1}
Outlook Classic	desktop	Microsoft Word	Unsupported^{See caveat 1}
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	Supported
Superhuman	desktop	Blink	Unknown

Documented caveats (1)

1.Outlook (New), Outlook Classic: Not supported, but the proprietary text-underline-style property can be used instead with the keyword value thick.

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

Client-by-client behaviour for text-decoration-thickness

Fully supports text-decoration-thickness (8): Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Proton Mail, Fastmail.

No support (12): 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. Plan fallbacks for these clients before relying on text-decoration-thickness in production sends.

Behaviour unverified in: Superhuman.