CSS Properties

aspect-ratio

CSS property · 5 supported, 0 partial, 16 unsupported across 21 clients

As of Sep 2, 2026, aspect-ratio fails or degrades in 16 of 21 email clients, treat it as progressive enhancement, never structure.

2/7 webmail, 2/6 desktop, 1/8 mobile clients support aspect-ratio fully.
Unsupported in Gmail, Gmail Android, Gmail iOS, Outlook 365 and 12 more.
Outlook Classic (Word engine, supported to at least 2029) does not fully support aspect-ratio, 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 Dec 19, 2023, synced by Emailens Sep 2, 2026.

aspect-ratio locks an element's height to a fraction of its width, preserving ratio across breakpoints. In email, support is recent and uneven: modern Apple Mail and webmail clients honor it, while Outlook and many mobile clients ignore it. Pre-computed width/height attributes remain the most reliable approach.

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
Outlook Classic	desktop	Microsoft Word	Unsupported
Outlook iOS	mobile	Outlook Mobile	Unsupported
Outlook Android	mobile	Outlook Mobile	Unsupported
Outlook for Mac	desktop	WebKit	Unsupported
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	Unsupported
Thunderbird	desktop	Gecko	Unsupported
HEY Mail	webmail	WebKit	Supported
Proton Mail	webmail	Proton	Supported^{See caveat 1}
AOL Mail	webmail	AOL	Unsupported
Fastmail	webmail	Fastmail	Unsupported
Superhuman	desktop	Blink	Supported

Documented caveats (1)

1.Proton Mail: Requires iOS 15.

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

Client-by-client behaviour for aspect-ratio

Fully supports aspect-ratio (5): Apple Mail, Apple Mail iOS, HEY Mail, Proton Mail, Superhuman.

No support (16): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook (New), Outlook Classic, Outlook iOS, Outlook Android, Outlook for Mac, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, Samsung Mail, Thunderbird, AOL Mail, Fastmail. Plan fallbacks for these clients before relying on aspect-ratio in production sends.

When to use aspect-ratio in email

Reserving space for hero imagery before the image loads.
Maintaining 16:9 video-thumbnail proportions in newsletter recap sections.
Square avatar tiles that need to scale fluidly with parent width.

Rendering behaviour and edge cases

Outlook on Windows ignores aspect-ratio entirely.
Gmail Web honors aspect-ratio in modern Chromium versions but Gmail mobile apps lag behind.
Apple Mail respects aspect-ratio reliably across recent macOS and iOS releases.

Recommended fallback strategy

Set explicit width and height attributes on <img> elements so blocked or slow-loading images still reserve their space. Use aspect-ratio as a progressive enhancement on container <div>s where it can avoid layout shift in modern clients.