# Email client support What `@cascivo/email` can and cannot use, derived from the [Can I email](https://www.caniemail.com) support matrix rather than asserted by hand. This page and the conformance lint read the same data, so they cannot disagree. **Snapshot:** Can I email api 1.0.4, tested 2026-08-10 15:16:24 +0000. Refresh with `pnpm email:caniemail:refresh` and commit the diff. ## How to read the verdict | Symbol | Meaning | | ------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------- | | ✅ | Supported by every client in the floor. | | ⚠️ | Partial in at least one client — usually "works on a table cell, not a div". Allowed, and enforced by structural invariants rather than by feature support. | | ❌ | Not supported by at least one client. The lint blocks it. | | — | Not in the matrix. Foundational HTML like `` and `` lands here; it is not a failure. | ## The support floor A feature must work in all of these for `@cascivo/email` to use it: - Outlook (Windows) - Outlook.com - Gmail (desktop webmail) - Gmail (Android) - Gmail (iOS) - Apple Mail (macOS) - Apple Mail (iOS) - Yahoo! Mail (desktop webmail) Outlook (Windows) is the binding constraint. It renders with the Word engine, it fails most modern CSS, and it is the one client that **cannot be tested in-house** — no emulator exists. It is in the floor precisely for that reason: the lint is what stands between the generator and a broken Outlook render. ## Feature support | Feature | Verdict | Outlook (Windows) | Outlook.com | Gmail (desktop webmail) | Gmail (Android) | Gmail (iOS) | Apple Mail (macOS) | Apple Mail (iOS) | Yahoo! Mail (desktop webmail) | | --------------------------- | ------- | ----------------- | ----------- | ----------------------- | --------------- | ----------- | ------------------ | ---------------- | ----------------------------- | | Flexbox | ❌ | ❌ | ✅ | ✅ | ⚠️ | ⚠️ | ✅ | ✅ | ✅ | | CSS Grid | ❌ | ❌ | ✅ | ✅ | ⚠️ | ⚠️ | ✅ | ✅ | ✅ | | `gap` | ❌ | ❌ | ⚠️ | ⚠️ | ⚠️ | ⚠️ | ✅ | ✅ | ❌ | | Custom properties (`var()`) | ❌ | ❌ | ❌ | ❌ | ❌ | ❌ | ✅ | ✅ | ❌ | | `oklch()` / `lab()` | ❌ | ❌ | ❌ | ❌ | ❌ | ❌ | ✅ | ✅ | ❌ | | `color-mix()` | — | — | — | — | — | — | — | — | — | | `rem` units | ❌ | ❌ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ❌ | | `border-radius` | ❌ | ❌ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ⚠️ | | `box-shadow` | ❌ | ❌ | ✅ | ❌ | ⚠️ | ⚠️ | ✅ | ✅ | ❌ | | `@media` | ❌ | ❌ | ⚠️ | ⚠️ | ⚠️ | ⚠️ | ✅ | ✅ | ⚠️ | | `prefers-color-scheme` | ❌ | ❌ | ✅ | ❌ | ❌ | ❌ | ✅ | ✅ | ❌ | | Inline SVG | ❌ | ❌ | ❌ | ❌ | ❌ | ❌ | ⚠️ | ✅ | ❌ | | `position` | ❌ | ❌ | ⚠️ | ❌ | ❌ | ❌ | ⚠️ | ⚠️ | ⚠️ | | `float` | ❌ | ❌ | ✅ | ✅ | ⚠️ | ⚠️ | ✅ | ✅ | ⚠️ | | `padding` | ⚠️ | ⚠️ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | | `background-color` | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | | `border-collapse` | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | | `vertical-align` | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | | `` | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ | ## Accepted exceptions A blocked feature may still be used when its absence **degrades rather than breaks** — the email stays legible, correct and actionable in the client that lacks it. This is the whole list: ### `css-at-media` Outlook Windows is the only floor client that reports `n`, and it is the one client that never needs the rule: it is desktop-only, and every media query this package emits narrows a fixed-width layout below a phone breakpoint. Absence there degrades to the same fixed 600px layout Outlook rendered before the rule existed. The alternative — no media query — is a 600px table in a 320px viewport, which is a measured 280px of sideways scroll for every mobile reader, so the trade is not close. ### `css-border-radius` Outlook Windows and Yahoo render square corners. Buttons, cards and badges stay fully legible and clickable — only the corner shape is lost. The alternative is a VML `v:roundrect` per button: ~400 bytes each, unable to inherit the anchor styles, and a second implementation of every button to keep in sync. Square corners in two clients is the better trade. ## What this does not tell you The matrix reports feature support. It does not reproduce a client's own rendering quirks, and no offline tool can: the Word engine is a proprietary Windows binary with no emulator. That residual is bought down by construction — the primitive set is built so it cannot emit what Outlook lacks — plus a release-time manual pass. See `docs/specs/email-target.md` §4.2 and §5.