HEY Mail vs Gmail: CSS Support Differences

Works in HEY Mail, breaks in Gmail

Each of these needs a fallback if the send reaches both.

• @font-face
• @import
• @keyframes
• @media device-pixel-ratio
• @media prefers-reduced-motion
• @supports
• accent-color
• align-items
• animation
• aspect-ratio
• block-size
• border-image
• bottom
• box-shadow
• clip-path
• conic-gradient
• custom-properties
• filter
• flex
• flex-basis
• flex-direction
• flex-wrap
• grid-template-columns
• inline-size
• justify-content
• left
• list-style-image
• margin-inline-block
• modern-color
• outline-offset
• pseudo-class-active
• pseudo-class-nth-last-of-type
• right
• scroll-snap
• text-decoration-skip-ink
• text-decoration-thickness
• text-shadow
• text-underline-offset
• top
• transform
• visibility
• z-index
• ::after
• ::before
• ::first-letter
• ::first-line
• ::marker
• :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 HEY Mail

• <abbr>
• <button type="reset">
• <button type="submit">
• <form>
• <input type="checkbox">
• <input type="hidden">
• <input type="radio">
• <input type="reset">
• <input type="submit">
• <input type="text">
• <select>
• <strike>
• <textarea>
• <wbr>
• [aria-hidden]
• [aria-label]
• [aria-live]
• [background]
• [height]
• [lang]
• [role]
• [width]
• amp4email
• anchor-links
• bimi
• doctype
• image-maps

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.

• <base>
• <blockquote>
• <body>
• <style>
• <ul>
• [align]
• [loading]
• @media
• !important
• gap
• html5-semantics
• light-dark
• list-style
• margin
• overflow
• position
• rgb
• rgba
• text-indent
• transition
• unit-initial
• word-break
• :attribute-selector

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 47
• HTML Elements 19
• Pseudo-Classes & Selectors 17
• HTML Attributes 10
• CSS At-Rules 7
• Pseudo-Elements 5

13 of these already have a fix

Each links to a copy-paste snippet that keeps the feature working in Gmail without dropping it in HEY Mail.

• @font-face
• animation
• box-shadow
• text-shadow
• transform
• visibility
• <style>
• @media
• gap
• overflow
• position
• transition
• word-break

Check your own HTML against both

Questions

How different are HEY Mail and Gmail?

They disagree on 111 of the 236 features verified in both clients, or 47%. 99 features render in both and are the safe intersection to build on.

What works in HEY Mail but not Gmail?

61 features, including @font-face, @import, @keyframes, @media device-pixel-ratio, @media prefers-reduced-motion and 56 more. Each needs a fallback if the send reaches both clients.

Which engines are these two clients using?

HEY Mail renders with WebKit; 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.