Engine Package
CSS Support Matrix
Which CSS properties are supported, partially supported, or unsupported in each email client.
The engine tracks support levels for 298 CSS and HTML features across all 21 email clients, auto-synced from caniemail.com data with manual overrides for HEY Mail and Superhuman.
| Level | Meaning |
|---|
supported | Fully works as expected |
partial | Limited or conditional support (see notes) |
unsupported | Does not work , will be stripped or ignored |
unknown | No data available |
| Property | Gmail | Outlook 365 | Outlook Win | Apple Mail | Yahoo | Samsung | Thunderbird |
|---|
display | Supported | Supported | Partial | Supported | Supported | Supported | Supported |
display: flex | Supported | Supported | Unsupported | Supported | Supported | Supported | Supported |
display: grid | Unsupported | Unsupported | Unsupported | Supported | Unsupported | Unsupported | Supported |
float | Supported | Supported | Unsupported | Supported | Partial | Supported | Supported |
position | Unsupported | Partial | Unsupported | Partial | Partial | Partial | Supported |
| Property | Gmail | Outlook 365 | Outlook Win | Apple Mail | Yahoo | Samsung | Thunderbird |
|---|
margin | Partial | Supported | Partial | Supported | Supported | Supported | Supported |
padding | Supported | Supported | Partial | Supported | Supported | Supported | Supported |
width | Supported | Supported | Supported | Supported | Supported | Supported | Supported |
max-width | Supported | Supported | Partial | Supported | Supported | Supported | Supported |
height | Supported | Supported | Supported | Supported | Supported | Supported | Supported |
box-sizing | Unsupported | Supported | Unsupported | Supported | Supported | Supported | Supported |
| Property | Gmail | Outlook 365 | Outlook Win | Apple Mail | Yahoo | Samsung | Thunderbird |
|---|
border | Supported | Supported | Partial | Supported | Supported | Supported | Supported |
border-radius | Supported | Supported | Unsupported | Supported | Partial | Supported | Supported |
| Property | Gmail | Outlook 365 | Outlook Win | Apple Mail | Yahoo | Samsung | Thunderbird |
|---|
background | Partial | Partial | Partial | Supported | Partial | Partial | Supported |
background-image | Supported | Supported | Partial | Supported | Partial | Supported | Supported |
color | Supported | Supported | Supported | Supported | Supported | Supported | Supported |
| Property | Gmail | Outlook 365 | Outlook Win | Apple Mail | Yahoo | Samsung | Thunderbird |
|---|
font-family | Supported | Supported | Supported | Supported | Supported | Supported | Supported |
font-size | Supported | Supported | Supported | Supported | Supported | Supported | Supported |
line-height | Supported | Supported | Partial | Supported | Supported | Supported | Supported |
text-decoration | Supported | Supported | Supported | Supported | Supported | Supported | Supported |
| Property | Gmail | Outlook 365 | Outlook Win | Apple Mail | Yahoo | Samsung | Thunderbird |
|---|
box-shadow | Unsupported | Supported | Unsupported | Supported | Unsupported | Supported | Supported |
opacity | Unsupported | Supported | Unsupported | Supported | Supported | Supported | Supported |
filter | Unsupported | Unsupported | Unsupported | Supported | Unsupported | Unsupported | Supported |
transform | Unsupported | Unsupported | Unsupported | Supported | Unsupported | Unsupported | Supported |
animation | Unsupported | Unsupported | Unsupported | Supported | Unsupported | Unsupported | Unsupported |
transition | Unsupported | Unsupported | Unsupported | Supported | Unsupported | Unsupported | Unsupported |
- 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 298-feature 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.