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.
| 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 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.