How different are Apple Mail and Outlook Classic?

They disagree on 242 of the 299 features verified in both clients, or 81%. 51 features render in both and are the safe intersection to build on.

What works in Apple Mail but not Outlook Classic?

205 features, including , , , , and 200 more. Each needs a fallback if the send reaches both clients.

Which engines are these two clients using?

Apple Mail renders with WebKit; Outlook Classic renders with Microsoft Word. 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 0.18.22 and last synced Nov 10, 2026. That sync date is not a test date: caniemail's own per-feature tests run from Feb 28, 2019 to Sep 16, 2026. Anything described as a transform is applied per those documented rules rather than captured from a real client.

Apple Mail vs Outlook Classic: CSS Support Differences

Apple Mail renders with WebKit; Outlook Classic renders with Microsoft Word. The gap runs almost entirely one way: 205 features work in Apple Mail and break in the other, against 0 in the reverse direction. Build for the stricter one and the permissive one follows.

Support data derived from caniemail.com (MIT) - last synced Nov 10, 2026 - engine data v0.18.22

work in both

the safe intersection

205

only in Apple Mail

breaks in Outlook Classic

only in Outlook Classic

breaks in Apple Mail

partial on one side

renders, with caveats

Works in Apple Mail, breaks in Outlook Classic

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

Works in Outlook Classic, breaks in Apple Mail

Nothing: every feature Outlook Classic supports is also supported in Apple Mail.

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 6 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 299.

1 of 300 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 141
HTML Elements 31
Pseudo-Classes & Selectors 26
HTML Attributes 17
CSS At-Rules 9
CSS Functions 8

33 of these already have a fix

Each links to a copy-paste snippet that keeps the feature working in Outlook Classic without dropping it in Apple Mail.

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Questions

How different are Apple Mail and Outlook Classic?: They disagree on 242 of the 299 features verified in both clients, or 81%. 51 features render in both and are the safe intersection to build on.
What works in Apple Mail but not Outlook Classic?: 205 features, including <abbr>, <acronym>, <audio>, <base>, <bdi> and 200 more. Each needs a fallback if the send reaches both clients.
Which engines are these two clients using?: Apple Mail renders with WebKit; Outlook Classic renders with Microsoft Word. 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 0.18.22 and last synced Nov 10, 2026. That sync date is not a test date: caniemail's own per-feature tests run from Feb 28, 2019 to Sep 16, 2026. Anything described as a transform is applied per those documented rules rather than captured from a real client.

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