How different are Gmail and Outlook 365?

They disagree on 77 of the 298 features verified in both clients, or 26%. 136 features render in both and are the safe intersection to build on.

What works in Gmail but not Outlook 365?

13 features, including , , [aria-live], amp4email, bimi and 8 more. Each needs a fallback if the send reaches both clients.

Which engines are these two clients using?

Gmail renders with Gmail Web; Outlook 365 renders with Outlook 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.

Gmail vs Outlook 365: CSS Support Differences

Gmail renders with Gmail Web; Outlook 365 renders with Outlook Web. Both clients break features the other renders, on 26% of the 298 features verified in both. There is no "stricter" client here to build against, so the safe set is the 136-feature intersection.

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

136

work in both

the safe intersection

only in Gmail

breaks in Outlook 365

only in Outlook 365

breaks in Gmail

partial on one side

renders, with caveats

Works in Gmail, breaks in Outlook 365

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 85 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 298.

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 28
Pseudo-Classes & Selectors 17
HTML Elements 12
CSS Functions 6
CSS At-Rules 5
Pseudo-Elements 5

8 of these already have a fix

Each links to a copy-paste snippet that keeps the feature working in Outlook 365 without dropping it in Gmail.

Check your own HTML against both

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Questions

How different are Gmail and Outlook 365?: They disagree on 77 of the 298 features verified in both clients, or 26%. 136 features render in both and are the safe intersection to build on.
What works in Gmail but not Outlook 365?: 13 features, including <button type="submit">, <input type="submit">, [aria-live], amp4email, bimi and 8 more. Each needs a fallback if the send reaches both clients.
Which engines are these two clients using?: Gmail renders with Gmail Web; Outlook 365 renders with Outlook 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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