Proton Mail vs Gmail: CSS Support Differences

Works in Proton Mail, breaks in Gmail

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

• <bdi>
• <meter>
• <progress>
• <rp>
• <rt>
• <ruby>
• <svg>
• [aria-describedby]
• [aria-labelledby]
• [hidden]
• [popover]
• @media hover
• align-items
• aspect-ratio
• backdrop-filter
• block-size
• border-image
• border-inline
• border-inline-individual
• border-inline-longhand
• border-radius-logical
• bottom
• box-shadow
• clip-path
• conic-gradient
• custom-properties
• filter
• flex
• flex-basis
• flex-direction
• flex-wrap
• grid-template-columns
• hyphenate-character
• hyphenate-limit-chars
• hyphens
• inline-size
• justify-content
• left
• margin-block-start-end
• margin-inline-block
• margin-inline-start-end
• max-block-size
• max-inline-size
• min-block-size
• min-inline-size
• orphans
• outline-offset
• padding-block-start-end
• padding-inline-block
• padding-inline-start-end
• position
• pseudo-class-active
• right
• scroll-snap
• shape-margin
• shape-outside
• tab-size
• text-decoration-skip-ink
• text-decoration-thickness
• text-emphasis-position
• text-shadow
• text-underline-offset
• text-wrap
• top
• transform
• visibility
• white-space-collapse
• widows
• z-index
• :focus
• :focus-visible
• :focus-within
• :lang

Works in Gmail, breaks in Proton Mail

• <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">
• @media orientation
• amp4email
• anchor-links
• bimi
• doctype
• image-maps
• min-height
• text-decoration
• :adjacent-sibling
• :chaining
• :child-combinator
• :class-selector
• :descendant-combinator
• :general-sibling
• :grouping
• :id-selector
• :type-selector
• :universal-selector

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>
• <body>
• <marquee>
• <style>
• <textarea>
• <ul>
• [align]
• @media
• @media prefers-reduced-motion
• !important
• cursor
• gap
• html5-semantics
• light-dark
• list-style
• mailto-links
• margin
• mask-image
• nesting
• overflow
• overflow-wrap
• rgb
• rgba
• system-ui
• text-indent
• text-justify
• unit-initial
• user-select
• 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 76
• HTML Elements 22
• Pseudo-Classes & Selectors 16
• HTML & Email Structure 6
• HTML Attributes 5
• CSS At-Rules 4

12 of these already have a fix

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

• <svg>
• box-shadow
• position
• text-shadow
• transform
• visibility
• <style>
• @media
• gap
• overflow
• overflow-wrap
• word-break

Check your own HTML against both

Questions

How different are Proton Mail and Gmail?

They disagree on 130 of the 300 features verified in both clients, or 43%. 120 features render in both and are the safe intersection to build on.

What works in Proton Mail but not Gmail?

73 features, including <bdi>, <meter>, <progress>, <rp>, <rt> and 68 more. Each needs a fallback if the send reaches both clients.

Which engines are these two clients using?

Proton Mail renders with Proton; 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.