CSS Properties

mask-image

CSS property · 4 supported, 1 partial, 12 unsupported across 21 clients

mask-image applies a mask layer to an element, hiding portions based on alpha or luminance. Email support is extremely limited , only WebKit-based Apple Mail and a few experimental clients honor it. For reliable cross-client masking effects, pre-render the masked image as a PNG.

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	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	Unknown
HEY Mail	webmail	WebKit	Unknown
Proton Mail	webmail	Proton	Partial
AOL Mail	webmail	AOL	Unsupported
Fastmail	webmail	Fastmail	Unknown
Superhuman	desktop	Blink	Unknown

Client-by-client behaviour for mask-image

Fully supports mask-image (4): Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail.

Partial support (1): Proton 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 (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 mask-image in production sends.

Behaviour unverified in: Thunderbird, HEY Mail, Fastmail, Superhuman.

When to use mask-image in email

Decorative shape masks in modern email designs targeting Apple Mail audiences.
Soft-edge image fades on hero banners viewed primarily on iOS devices.
Texture overlays for premium brand newsletters where the audience skews Apple Mail / Superhuman.

Rendering behaviour and edge cases

Outlook, Gmail, and Yahoo all ignore mask-image , design must remain visually intact without it.
Even in Apple Mail, mask-image performance varies on older iOS versions; test before deploying.
Combining mask-image with animation or transform produces unpredictable rendering across clients.

Recommended fallback strategy

Pre-render any masked imagery as a PNG with the mask already baked in. Reserve mask-image for purely decorative effects layered on top of an already-complete design.