Emailens
Engine Package

CSS Support Matrix

Which CSS properties are supported, partially supported, or unsupported in each email client.

The engine tracks support levels for 250+ CSS properties across all 13 email clients, auto-synced from caniemail.com data with manual overrides for HEY Mail and Superhuman.

Support levels

LevelMeaning
supportedFully works as expected
partialLimited or conditional support (see notes)
unsupportedDoes not work — will be stripped or ignored
unknownNo data available

Layout properties

PropertyGmailOutlook 365Outlook WinApple MailYahooSamsungThunderbird
displaySupportedSupportedPartialSupportedSupportedSupportedSupported
display: flexSupportedSupportedUnsupportedSupportedSupportedSupportedSupported
display: gridUnsupportedUnsupportedUnsupportedSupportedUnsupportedUnsupportedSupported
floatSupportedSupportedUnsupportedSupportedPartialSupportedSupported
positionUnsupportedPartialUnsupportedPartialPartialPartialSupported

Box model

PropertyGmailOutlook 365Outlook WinApple MailYahooSamsungThunderbird
marginPartialSupportedPartialSupportedSupportedSupportedSupported
paddingSupportedSupportedPartialSupportedSupportedSupportedSupported
widthSupportedSupportedSupportedSupportedSupportedSupportedSupported
max-widthSupportedSupportedPartialSupportedSupportedSupportedSupported
heightSupportedSupportedSupportedSupportedSupportedSupportedSupported
box-sizingUnsupportedSupportedUnsupportedSupportedSupportedSupportedSupported

Borders

PropertyGmailOutlook 365Outlook WinApple MailYahooSamsungThunderbird
borderSupportedSupportedPartialSupportedSupportedSupportedSupported
border-radiusSupportedSupportedUnsupportedSupportedPartialSupportedSupported

Colors & backgrounds

PropertyGmailOutlook 365Outlook WinApple MailYahooSamsungThunderbird
backgroundPartialPartialPartialSupportedPartialPartialSupported
background-imageSupportedSupportedPartialSupportedPartialSupportedSupported
colorSupportedSupportedSupportedSupportedSupportedSupportedSupported

Text

PropertyGmailOutlook 365Outlook WinApple MailYahooSamsungThunderbird
font-familySupportedSupportedSupportedSupportedSupportedSupportedSupported
font-sizeSupportedSupportedSupportedSupportedSupportedSupportedSupported
line-heightSupportedSupportedPartialSupportedSupportedSupportedSupported
text-decorationSupportedSupportedSupportedSupportedSupportedSupportedSupported

Effects

PropertyGmailOutlook 365Outlook WinApple MailYahooSamsungThunderbird
box-shadowUnsupportedSupportedUnsupportedSupportedUnsupportedSupportedSupported
opacityUnsupportedSupportedUnsupportedSupportedSupportedSupportedSupported
filterUnsupportedUnsupportedUnsupportedSupportedUnsupportedUnsupportedSupported
transformUnsupportedUnsupportedUnsupportedSupportedUnsupportedUnsupportedSupported
animationUnsupportedUnsupportedUnsupportedSupportedUnsupportedUnsupportedUnsupported
transitionUnsupportedUnsupportedUnsupportedSupportedUnsupportedUnsupportedUnsupported

Key takeaways

  • Gmail supports flexbox and border-radius but strips grid, transforms, animations, and box-shadow.
  • Outlook Classic uses the Microsoft Word rendering engine. No border-radius, flexbox, grid, box-shadow, or opacity. The new Outlook for Windows uses the same web engine as Outlook 365 with much better CSS support.
  • Apple Mail has the best CSS support (WebKit engine) — supports nearly everything.
  • Thunderbird (Gecko) also has excellent CSS support.
  • Samsung Mail has good overall support including flexbox, @font-face, and box-shadow, but lacks grid, transforms, and animations.
  • Yahoo Mail falls between Gmail and Apple Mail in support level.

For the full 250+ property list and per-client notes, see the engine source at src/rules/css-support.ts, auto-generated from caniemail.com data via bun run sync:caniemail.

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