How different are HEY Mail and Gmail?

They disagree on 109 of the 234 features verified in both clients, or 47%. 99 features render in both and are the safe intersection to build on.

What works in HEY Mail but not Gmail?

59 features, including @font-face, @import, @keyframes, @media device-pixel-ratio, @media prefers-reduced-motion and 54 more. Each needs a fallback if the send reaches both clients.

Which engines are these two clients using?

HEY Mail renders with WebKit; Gmail renders with Gmail Web. Where two clients share an engine the divergence is usually configuration, and where they do not it is the engine itself.

Is this measured in the real clients?

No. These are documented support levels derived from caniemail.com (MIT-licensed), carried in @emailens/engine data version 0.12.2 and last synced Sep 2, 2026. That sync date is not a test date: caniemail's own per-feature tests run from Feb 28, 2019 to Jul 13, 2026. Anything described as a transform is applied per those documented rules rather than captured from a real client.

HEY Mail vs Gmail: CSS Support Differences

HEY Mail renders with WebKit; Gmail renders with Gmail Web. Both clients break features the other renders, on 47% of the 234 features verified in both. There is no "stricter" client here to build against, so the safe set is the 99-feature intersection.

Support data derived from caniemail.com (MIT) - last synced Sep 2, 2026 - engine data v0.12.2

work in both

the safe intersection

only in HEY Mail

breaks in Gmail

only in Gmail

breaks in HEY Mail

partial on one side

renders, with caveats

Works in HEY Mail, breaks in Gmail

Each of these needs a fallback if the send reaches both.

Partial on one side

These render in both, but one client applies caveats the other does not. They fail quietly, which makes them the expensive ones.

A further 26 features render in neither client. Not a difference between them, but the other half of the answer to what you can actually use, so the four counts above deliberately do not sum to 234.

64 of 298 features have no confident data point for at least one of these clients, so they are excluded from every count above rather than assumed either way.

Where the divergence sits

Two pairs with the same headline number can be different problems. For these two it concentrates in CSS Properties.

CSS Properties 45
HTML Elements 19
Pseudo-Classes & Selectors 17
HTML Attributes 10
CSS At-Rules 7
Pseudo-Elements 5

13 of these already have a fix

Each links to a copy-paste snippet that keeps the feature working in Gmail without dropping it in HEY Mail.

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Questions

How different are HEY Mail and Gmail?: They disagree on 109 of the 234 features verified in both clients, or 47%. 99 features render in both and are the safe intersection to build on.
What works in HEY Mail but not Gmail?: 59 features, including @font-face, @import, @keyframes, @media device-pixel-ratio, @media prefers-reduced-motion and 54 more. Each needs a fallback if the send reaches both clients.
Which engines are these two clients using?: HEY Mail renders with WebKit; Gmail renders with Gmail Web. Where two clients share an engine the divergence is usually configuration, and where they do not it is the engine itself.
Is this measured in the real clients?: No. These are documented support levels derived from caniemail.com (MIT-licensed), carried in @emailens/engine data version 0.12.2 and last synced Sep 2, 2026. That sync date is not a test date: caniemail's own per-feature tests run from Feb 28, 2019 to Jul 13, 2026. Anything described as a transform is applied per those documented rules rather than captured from a real client.

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