The State of Email CSS

01The client leaderboard

Each bar shows how all 298 features behave in one client. Percentages are the share of tested features with full support; the neutral segment is features not yet verified for that client.

02Outlook is not one client

The Outlook brand spans five apps and two rendering worlds. Outlook on iOS renders 62% of features fully; Outlook for Windows renders 19%, because it still lays out email with Microsoft Word's engine. 91 features work in Outlook 365 on the web but fail in Outlook for Windows. When someone says "test it in Outlook," ask which one.

03The Outlook traps

These 84 features work in nearly every other client, which is exactly what makes them dangerous: your preview looks fine everywhere you check, then Outlook for Windows drops them silently.

04Gmail is not one client either

54 features that work in Gmail on the web degrade or disappear in the Gmail mobile apps, mostly when the account is a non-Google (IMAP) account. The casualties include things most developers assume are safe: float, calc(), linear-gradient(), white-space, and every class and ID selector, which fall back to partial support when embedded styles are stripped.

05The safe list

Fully supported in all 21 clients:

Widen the bar to "supported or partial everywhere" and the list grows to 21, which is the practical foundation of every email layout in production today:

06Methodology

Support classifications are based on caniemail.com community testing data, as compiled and consumed by our open-source @emailens/engine package (MIT). Each of the 298 features (CSS properties, HTML elements, selectors, functions, units and at-rules) is classified per client as supported, partially supported, or unsupported. Per-feature details live in the email CSS reference. Snapshot: July 2026.

Emailens, "The State of Email CSS" (July 2026), https://emailens.dev/email-css/report

07Questions

How many CSS features work in every email client?

6 of 298 tracked features are supported across all 21 clients in this report, synced Sep 2, 2026. Allowing for clients with no confident data point, the practical safe list is 21.

Which email client supports the least CSS?

Outlook Classic, which renders with Microsoft Word rather than a browser engine. Microsoft supports it to at least 2029, so its constraints are not going away on any near-term schedule.

Where does the data come from, and how current is it?

Support levels derive from caniemail.com (MIT-licensed), carried in @emailens/engine data version 0.12.2 and last synced Sep 2, 2026. That date is anchored to a content hash of the matrix, so it moves only when the data itself changes, and it is a sync rather than a test: caniemail's own per-feature test dates run from Feb 28, 2019 to Jul 13, 2026, and each feature page shows the one for its feature.

Is this measured in real email clients?

No. Support levels are documented client behaviour, and anything this report calls a transform or a simulation is applied per those documented rules rather than captured from a real client. Claims are scoped accordingly.