The State of Email CSS

01The client leaderboard

Each bar shows how all 251 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 60% of features fully; Outlook for Windows renders 18%, because it still lays out email with Microsoft Word's engine. 78 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 28 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

45 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 15 clients:

Widen the bar to "supported or partial everywhere" and the list grows to 23, 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 251 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