Fastmail vs Gmail: CSS Support Differences
Fastmail renders with Fastmail; Gmail renders with Gmail Web. The gap runs almost entirely one way: 61 features work in Fastmail and break in the other, against 9 in the reverse direction. Build for the stricter one and the permissive one follows.
Support data derived from caniemail.com (MIT) - last synced Nov 10, 2026 - engine data v0.18.22
119
work in both
the safe intersection
61
only in Fastmail
breaks in Gmail
9
only in Gmail
breaks in Fastmail
18
partial on one side
renders, with caveats
Works in Fastmail, breaks in Gmail
Each of these needs a fallback if the send reaches both.
- <bdi>
- <meter>
- <progress>
- <rp>
- <rt>
- <ruby>
- [required]
- @keyframes
- @media device-pixel-ratio
- @media prefers-color-scheme
- align-items
- animation
- block-size
- border-image
- bottom
- box-shadow
- clip-path
- conic-gradient
- flex
- flex-basis
- flex-direction
- flex-wrap
- justify-content
- left
- list-style-image
- margin-inline-block
- modern-color
- outline-offset
- pseudo-class-active
- pseudo-class-nth-last-of-type
- right
- text-decoration-skip-ink
- text-decoration-thickness
- text-shadow
- text-underline-offset
- top
- transform
- transition
- visibility
- z-index
- ::after
- ::before
- ::first-letter
- ::first-line
- ::marker
- ::placeholder
- :checked
- :first-child
- :first-of-type
- :focus
- :has
- :last-child
- :last-of-type
- :link
- :not
- :nth-child
- :nth-last-child
- :nth-of-type
- :only-child
- :only-of-type
- :visited
Works in Gmail, breaks in Fastmail
Partial on one side
These render in both, but one client applies caveats the other does not. They fail quietly, which makes them the expensive ones.
A further 28 features render in neither client. Not a difference between them, but the other half of the answer to what you can actually use, so the four counts above deliberately do not sum to 235.
65 of 300 features have no confident data point for at least one of these clients, so they are excluded from every count above rather than assumed either way.
Where the divergence sits
Two pairs with the same headline number can be different problems. For these two it concentrates in CSS Properties.
- CSS Properties 42
- Pseudo-Classes & Selectors 18
- HTML Elements 11
- Pseudo-Elements 6
- CSS At-Rules 4
- CSS Functions 4
11 of these already have a fix
Each links to a copy-paste snippet that keeps the feature working in Gmail without dropping it in Fastmail.
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Questions
- How different are Fastmail and Gmail?
- They disagree on 88 of the 235 features verified in both clients, or 37%. 119 features render in both and are the safe intersection to build on.
- What works in Fastmail but not Gmail?
- 61 features, including <bdi>, <meter>, <progress>, <rp>, <rt> and 56 more. Each needs a fallback if the send reaches both clients.
- Which engines are these two clients using?
- Fastmail renders with Fastmail; Gmail renders with Gmail Web. Where two clients share an engine the divergence is usually configuration, and where they do not it is the engine itself.
- Is this measured in the real clients?
- No. These are documented support levels derived from caniemail.com (MIT-licensed), carried in @emailens/engine data version 0.18.22 and last synced Nov 10, 2026. That sync date is not a test date: caniemail's own per-feature tests run from Feb 28, 2019 to Sep 16, 2026. Anything described as a transform is applied per those documented rules rather than captured from a real client.
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