CSS Functions

clamp

CSS function · 6 supported, 3 partial, 11 unsupported across 21 clients

The clamp() function locks a value between a minimum and maximum, with a preferred middle value. It is widely used in modern responsive web design but has limited email support , Apple Mail and modern Gmail Web honor it, while Outlook on Windows ignores it entirely.

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Supported
Gmail Android	mobile	Gmail Mobile	Partial
Gmail iOS	mobile	Gmail Mobile	Partial
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	Partial
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Supported
Proton Mail	webmail	Proton	Supported
AOL Mail	webmail	AOL	Unsupported
Fastmail	webmail	Fastmail	Unsupported
Superhuman	desktop	Blink	Unknown

Client-by-client behaviour for clamp

Fully supports clamp (6): Gmail, Apple Mail, Apple Mail iOS, Thunderbird, HEY Mail, Proton Mail.

Partial support (3): Gmail Android, Gmail iOS, Samsung Mail. Expect rendering quirks unique to each engine. Partial support typically means a subset of values, an ignored shorthand, or sanitizer-specific rewrites.

No support (11): Outlook 365, Outlook (New), Outlook Classic, Outlook iOS, Outlook Android, Outlook for Mac, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail, Fastmail. Plan fallbacks for these clients before relying on clamp in production sends.

Behaviour unverified in: Superhuman.

When to use clamp in email

Fluid typography that scales with viewport width while staying within readable bounds.
Responsive container padding that adapts to small and large screens.
Adaptive button sizing in modern marketing email templates.

Rendering behaviour and edge cases

Outlook on Windows ignores clamp() and uses the original CSS keyword fallback.
Gmail Web supports clamp() in recent Chromium versions; Gmail mobile is more conservative.
Browser-style fluid typography expectations don't always translate to email viewports.

Recommended fallback strategy

Provide a fixed pixel fallback declaration before clamp() so non-supporting clients still receive a sensible value. Keep the clamp() range conservative in email , extreme min/max values render unpredictably across mobile clients.