Fastmail
CSS coverage profile for Fastmail
Fastmail renders with the Fastmail engine and supports 191 of 298 tracked CSS and HTML features fully, with 4 partially supported and 38 unsupported. That places it in a moderate coverage band.
A meaningful portion of CSS is unsupported or partially supported. Plan fallbacks for layout-critical features and validate every template before send.
191
Supported
4
Partial
38
Unsupported
298
Total
Breaks here, works nearly everywhere else
2 features that Fastmail drops while most other clients render them. This is usually what is behind an email that looks correct everywhere but here.
Renders here, dropped almost everywhere else
Safe in Fastmail and almost nowhere else, so it needs a fallback the moment the send is not Fastmail-only.
Where Fastmail is weakest
- CSS Properties 15
- HTML Elements 11
- HTML Attributes 6
- CSS At-Rules 4
- CSS Functions 3
Closest client: Apple Mail
Apple Mail agrees with Fastmail on 82% of verified features. They still differ on 42: <audio>, <base>, <body>, <dialog>, <form>, <link>, and 36 more.
Side by side
HTML Elements48
| Feature | Support |
|---|---|
| <abbr> | Supported |
| <acronym> | Unknown |
| <address> | Supported |
| <audio> | Unsupported |
| <base> | Partial |
| <bdi> | Supported |
| <blockquote> | Supported |
| <body> | Unsupported |
| <button type="reset"> | Supported |
| <button type="submit"> | Supported |
| <code> | Supported |
| <del> | Supported |
| <dfn> | Unknown |
| <dialog> | Unsupported |
| <div> | Supported |
| <form> | Unsupported |
| <h1> | Supported |
| <hr> | Unknown |
| <img> | Supported |
| <input type="checkbox"> | Supported |
| <input type="hidden"> | Supported |
| <input type="radio"> | Supported |
| <input type="reset"> | Supported |
| <input type="submit"> | Supported |
| <input type="text"> | Supported |
| <link> | Unsupported |
| <marquee> | Unsupported |
| <meter> | Supported |
| <object> | Unsupported |
| <p> | Supported |
| <picture> | Unsupported |
| <pre> | Supported |
| <progress> | Supported |
| <rp> | Supported |
| <rt> | Supported |
| <ruby> | Supported |
| <select> | Supported |
| <small> | Supported |
| <span> | Supported |
| <strike> | Supported |
| <strong> | Supported |
| <style> | Supported |
| <svg> | Unsupported |
| <table> | Supported |
| <textarea> | Supported |
| <ul> | Supported |
| <video> | Unsupported |
| <wbr> | Supported |
HTML Attributes22
| Feature | Support |
|---|---|
| [align] | Supported |
| [aria-describedby] | Unsupported |
| [aria-hidden] | Supported |
| [aria-label] | Supported |
| [aria-labelledby] | Unsupported |
| [aria-live] | Supported |
| [background] | Supported |
| [cellpadding] | Unknown |
| [cellspacing] | Unknown |
| [command] | Unknown |
| [dir] | Supported |
| [height] | Supported |
| [hidden] | Unknown |
| [lang] | Supported |
| [loading] | Unsupported |
| [popover] | Unsupported |
| [required] | Supported |
| [role] | Supported |
| [srcset] | Unsupported |
| [target] | Unsupported |
| [valign] | Supported |
| [width] | Supported |
HTML & Email Structure8
| Feature | Support |
|---|---|
| amp4email | Unsupported |
| anchor-links | Supported |
| doctype | Supported |
| html-comments | Unknown |
| html5-semantics | Supported |
| image-maps | Supported |
| mailto-links | Supported |
| meta-color-scheme | Unsupported |
CSS At-Rules10
| Feature | Support |
|---|---|
| @font-face | Unsupported |
| @import | Unsupported |
| @keyframes | Supported |
| @media | Supported |
| @media device-pixel-ratio | Supported |
| @media hover | Unknown |
| @media orientation | Supported |
| @media prefers-color-scheme | Supported |
| @media prefers-reduced-motion | Unsupported |
| @supports | Unsupported |
Pseudo-Elements6
| Feature | Support |
|---|---|
| ::after | Supported |
| ::before | Supported |
| ::first-letter | Supported |
| ::first-line | Supported |
| ::marker | Supported |
| ::placeholder | Supported |
Pseudo-Classes & Selectors34
| Feature | Support |
|---|---|
| :adjacent-sibling | Supported |
| :attribute-selector | Supported |
| :chaining | Supported |
| :checked | Supported |
| :child-combinator | Supported |
| :class-selector | Supported |
| :descendant-combinator | Supported |
| :first-child | Supported |
| :first-of-type | Supported |
| :focus | Supported |
| :focus-visible | Unknown |
| :focus-within | Unknown |
| :general-sibling | Supported |
| :grouping | Supported |
| :has | Supported |
| :hover | Supported |
| :id-selector | Supported |
| :lang | Unknown |
| :last-child | Supported |
| :last-of-type | Supported |
| :link | Supported |
| :not | Supported |
| :nth-child | Supported |
| :nth-last-child | Supported |
| :nth-of-type | Supported |
| :only-child | Supported |
| :only-of-type | Supported |
| :target | Unsupported |
| :type-selector | Supported |
| :universal-selector | Supported |
| :visited | Supported |
| pseudo-class-active | Supported |
| pseudo-class-default | Unknown |
| pseudo-class-nth-last-of-type | Supported |
CSS Functions8
| Feature | Support |
|---|---|
| calc | Supported |
| clamp | Unsupported |
| conic-gradient | Supported |
| fit-content | Supported |
| linear-gradient | Supported |
| max | Unsupported |
| min | Unsupported |
| radial-gradient | Supported |
CSS Properties162
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Support data last updated Sep 2, 2026 - synced from caniemail.com (MIT) via @emailens/engine.