Fastmail email CSS: what breaks, and what to write instead

Fastmail is tested on 235 of 300 features: 193 work, 4 partial, 38 not supported. The other 65 are untested.

Works 193 · Partial 4 · Not supported 38 · Untested 65 · of 300 · Tested on fewer than 270 of 300 features

Fastmail · webmail · prefers-color-scheme: works

Check your email, free

No signup · Five clients free, Outlook Classic included · All 21 with an account

The checker needs a computer screen.

See what breaks ↓

Fastmail opens with a free account.

A free account opens all 21 clients and keeps your checks for 7 days. Create a free account →

@emailens/engine 0.18.22 · data last changed 10 Nov 2026

Where Fastmail is the outlier

2 features work in at least 14 of the other 20 clients and are not supported here.

So an email that looks right in most clients can reach your reader in Fastmail without them.

65 features are untested in Fastmail. They are left out here, not counted as not supported.

What works in Fastmail and breaks elsewhere

31 features work in Fastmail and in 6 or fewer of the other 20 clients, so they need a fallback.

65 features are untested in Fastmail. They are left out here, not counted as not supported.

overflow
Works in 4 of 21 · partial in 16 · not supported in 1
· works here
word-break
Works in 4 of 21 · partial in 12 · not supported in 5
· works here
animation
Works in 5 of 21 · partial in 1 · not supported in 15
· works here
modern-color
Works in 4 of 21 · not supported in 16 · 1 untested
· works here
Depends on browser support.
transition
Works in 4 of 21 · partial in 7 · not supported in 10
· works here
Transition properties are supported but pseudo-classes like :hover are not.
:has
Works in 5 of 21 · not supported in 15 · 1 untested
· works here
@media
Works in 5 of 21 · partial in 14 · not supported in 2
· works here
::after
Works in 6 of 21 · partial in 4 · not supported in 10 · 1 untested
· works here

All 31

What to write instead

4 features Fastmail does not fully support have a written fix. See them in the table ↓

<form>

The fix for Fastmail

Replace embedded form with a link to a hosted form

after

13 lines

<table role="presentation" cellpadding="0" cellspacing="0">
  <tr>
    <td style="background-color: #6d28d9; border-radius: 6px;
               padding: 12px 32px;">
      <a href="https://example.com/subscribe"
        style="color: #fff; text-decoration: none;
               font-family: Arial, sans-serif;
               font-weight: bold;">
        Subscribe Now
      </a>
    </td>
  </tr>
</table>

Show the original

before

4 lines

<form action="/subscribe" method="POST">
  <input type="email" placeholder="Email" />
  <button type="submit">Subscribe</button>
</form>

Test this fix, free

Fastmail opens with a free account; the checker shows five clients without one.

The checker needs a computer screen.

<svg>

The fix for Fastmail

Convert inline SVG to an image for email compatibility

after

3 lines

<img src="https://example.com/icon.png"
  width="24" height="24" alt="Icon"
  style="display: block; border: 0;" />

Show the original

before

3 lines

<svg width="24" height="24" viewBox="0 0 24 24">
  <path d="M12 2L2 7l10 5 10-5-10-5z" fill="#6d28d9"/>
</svg>

Test this fix, free

Fastmail opens with a free account; the checker shows five clients without one.

The checker needs a computer screen.

<video>

The fix for Fastmail

Replace video with animated GIF or linked thumbnail

after

5 lines

<a href="https://example.com/watch" target="_blank">
  <img src="https://example.com/video-thumb.gif"
    width="600" alt="Watch the video"
    style="display: block; border: 0; max-width: 100%;" />
</a>

Show the original

before

3 lines

<video width="600" autoplay muted>
  <source src="demo.mp4" type="video/mp4">
</video>

Test this fix, free

Fastmail opens with a free account; the checker shows five clients without one.

The checker needs a computer screen.

Paste your email and see what Fastmail does to it.

Check your email, free

No signup · Five clients free, Outlook Classic included · All 21 with an account

The checker needs a computer screen.

Dark mode in Fastmail

Works @media prefers-color-scheme works in Fastmail.

Works in 9 of 21.
Not supported color-scheme is not supported in Fastmail.

Works in 4 of 21.
Not supported meta-color-scheme is not supported in Fastmail.

Works in 2 of 21.

Your next question

Gmail · Outlook 365 · Yahoo Mail · HEY Mail · Proton Mail · AOL Mail

Every feature in Fastmail

What breaks here (42) Everything (300) Untested (65)

HTML Elements · 11 of 48 not supported or partial here

<abbr> Works Works in 14 of 21
<acronym> Untested Works in 13 of 21
<address> Works Works in 20 of 21
<audio> Not supported. Not supported Works in 4 of 21
<base> Buggy. Partial Works in 4 of 21
<bdi> Works Works in 6 of 21
<blockquote> Works Works in 19 of 21
<body> Not supported Works in 5 of 21
<button type="reset"> Works Works in 10 of 21
<button type="submit"> Works Works in 11 of 21
<code> Works Works in 20 of 21
<del> Works Works in 20 of 21
<dfn> Untested Works in 17 of 21
<dialog> Not supported Works in 5 of 21
<div> Works Works in 20 of 21
<form> fix Not supported Works in 10 of 21
<h1> Works Works in 20 of 21
<hr> Untested Works in 18 of 21
<img> Works Works in 19 of 21
<input type="checkbox"> Works Works in 16 of 21
<input type="hidden"> Works Works in 16 of 21
<input type="radio"> Works Works in 16 of 21
<input type="reset"> Works Works in 15 of 21
<input type="submit"> Works Works in 12 of 21
<input type="text"> Works Works in 13 of 21
<link> fix Not supported Works in 3 of 21
<marquee> Not supported Works in 8 of 21
<meter> Works Works in 11 of 21
<object> Not supported. Not supported Works in 1 of 21
<p> Works Works in 20 of 21
<picture> Not supported Works in 4 of 21
<pre> Works Works in 20 of 21
<progress> Works Works in 6 of 21
<rp> Works Works in 15 of 21
<rt> Works Works in 15 of 21
<ruby> Works Works in 15 of 21
<select> Works Works in 14 of 21
<small> Works Works in 20 of 21
<span> Works Works in 20 of 21
<strike> Works Works in 17 of 21
<strong> Works Works in 20 of 21
<style> Works Works in 16 of 21
<svg> fix Not supported Works in 5 of 21
<table> Works Works in 20 of 21
<textarea> Works Works in 13 of 21
<ul> Works Works in 7 of 21
<video> fix Not supported. Not supported Works in 3 of 21
<wbr> Works Works in 16 of 21

HTML Attributes · 6 of 22 not supported or partial here

[align] Works Works in 17 of 21
[aria-describedby] Buggy. Not supported Works in 9 of 21
[aria-hidden] Works Works in 13 of 21
[aria-label] Works Works in 13 of 21
[aria-labelledby] Buggy. Not supported Works in 7 of 21
[aria-live] Works Works in 8 of 21
[background] Works Works in 16 of 21
[cellpadding] Untested Works in 18 of 21
[cellspacing] Untested Works in 18 of 21
[command] Untested Works in 3 of 21
[dir] Works Works in 19 of 21
[height] Works Works in 18 of 21
[hidden] Untested Works in 5 of 21
[lang] Works Works in 15 of 21
[loading] Not supported Works in 4 of 21
[popover] Not supported Works in 3 of 21
[required] Works Works in 7 of 21
[role] Works Works in 14 of 21
[srcset] The sizes attribute is supported but not srcset. Not supported Works in 4 of 21
[target] Not supported. Not supported Works in 0 of 21
[valign] Works Works in 20 of 21
[width] Works Works in 18 of 21

HTML & Email Structure · 2 of 8 not supported or partial here

amp4email Not supported Works in 6 of 21
anchor-links Works Works in 12 of 21
doctype Works Works in 13 of 21
html-comments Untested Works in 18 of 21
html5-semantics Works Works in 6 of 21
image-maps Works Works in 15 of 21
mailto-links Works Works in 17 of 21
meta-color-scheme Not supported Works in 2 of 21

CSS At-Rules · 4 of 10 not supported or partial here

@font-face Not supported Works in 6 of 21
@import Not supported Works in 4 of 21
@keyframes Works Works in 7 of 21
@media Works Works in 5 of 21
@media device-pixel-ratio Works Works in 10 of 21
@media hover Untested Works in 9 of 21
@media orientation Works Works in 11 of 21
@media prefers-color-scheme Works Works in 9 of 21
@media prefers-reduced-motion Not supported. Not supported Works in 9 of 21
@supports Not supported. Not supported Works in 9 of 21

Pseudo-Elements · 0 of 6 not supported or partial here

::after Works Works in 6 of 21
::before Works Works in 6 of 21
::first-letter Works Works in 10 of 21
::first-line Works Works in 10 of 21
::marker Works Works in 10 of 21
::placeholder Works Works in 9 of 21

Pseudo-Classes & Selectors · 1 of 34 not supported or partial here

:adjacent-sibling Works Works in 16 of 21
:attribute-selector Works Works in 6 of 21
:chaining Works Works in 14 of 21
:checked Works Works in 9 of 21
:child-combinator Works Works in 16 of 21
:class-selector Works Works in 17 of 21
:descendant-combinator Works Works in 17 of 21
:first-child Works Works in 12 of 21
:first-of-type Works Works in 12 of 21
:focus Works Works in 10 of 21
:focus-visible Untested Works in 6 of 21
:focus-within Untested Works in 6 of 21
:general-sibling Works Works in 16 of 21
:grouping Works Works in 17 of 21
:has Works Works in 5 of 21
:hover Works Works in 12 of 21
:id-selector Works Works in 17 of 21
:lang Untested Works in 6 of 21
:last-child Works Works in 12 of 21
:last-of-type Works Works in 12 of 21
:link Works Works in 13 of 21
:not Works Works in 6 of 21
:nth-child Works Works in 6 of 21
:nth-last-child Works Works in 6 of 21
:nth-of-type Works Works in 6 of 21
:only-child Works Works in 12 of 21
:only-of-type Works Works in 12 of 21
:target Buggy. Not supported Works in 7 of 21
:type-selector Works Works in 17 of 21
:universal-selector Works Works in 16 of 21
:visited Works Works in 8 of 21
pseudo-class-active Works Works in 8 of 21
pseudo-class-default Untested Works in 10 of 21
pseudo-class-nth-last-of-type Works Works in 6 of 21

CSS Functions · 3 of 8 not supported or partial here

calc Works Works in 8 of 21
clamp Not supported Works in 6 of 21
conic-gradient Works Works in 6 of 21
fit-content Works Works in 11 of 21
linear-gradient Works Works in 9 of 21
max Not supported Works in 6 of 21
min Not supported Works in 6 of 21
radial-gradient Works Works in 10 of 21

CSS Properties · 15 of 164 not supported or partial here

!important Works Works in 11 of 21
accent-color Not supported Works in 5 of 21
align-items Works Works in 13 of 21
animation Works Works in 5 of 21
aspect-ratio Not supported Works in 5 of 21
backdrop-filter Untested Works in 6 of 21
background Works Works in 16 of 21
background-blend-mode Works Works in 13 of 21
background-clip Works Works in 14 of 21
background-color Works Works in 21 of 21
background-image Works Works in 16 of 21
background-origin Works Works in 14 of 21
background-position Works Works in 20 of 21
background-repeat Works Works in 20 of 21
background-size Works Works in 17 of 21
bimi Works Works in 9 of 21
block-size Works Works in 7 of 21
border Works Works in 20 of 21
border-collapse Untested Works in 19 of 21
border-image Works Works in 7 of 21
border-inline Untested Works in 3 of 21
border-inline-individual Untested Works in 3 of 21
border-inline-longhand Untested Works in 4 of 21
border-radius Works Works in 16 of 21
border-radius-logical Untested Works in 3 of 21
border-spacing Untested Works in 17 of 21
bottom Works Works in 11 of 21
box-shadow Works Works in 14 of 21
box-sizing Works Works in 16 of 21
caption-side Works Works in 20 of 21
clear Untested Works in 9 of 21
clip-path Works Works in 8 of 21
color Works Works in 21 of 21
color-scheme Not supported Works in 4 of 21
column-count Works Works in 16 of 21
columns Works Works in 16 of 21
css-comments Untested Works in 14 of 21
cursor Untested Works in 2 of 21
custom-properties Partial. Not supported Works in 6 of 21
direction Works Works in 21 of 21
display Partial. Partial Works in 13 of 21
display:flex Works Works in 18 of 21
display:grid Works Works in 18 of 21
display:none Works Works in 20 of 21
empty-cells Untested Works in 14 of 21
filter Not supported Works in 7 of 21
flex Works Works in 13 of 21
flex-basis Works Works in 13 of 21
flex-direction Works Works in 12 of 21
flex-wrap Works Works in 13 of 21
float Works Works in 14 of 21
font Works Works in 20 of 21
font-family Works Works in 21 of 21
font-kerning Untested Works in 13 of 21
font-size Untested Works in 13 of 21
font-size-adjust Untested Works in 10 of 21
font-stretch Untested Works in 10 of 21
font-weight Works Works in 16 of 21
gap Partial. Partial Works in 7 of 21
grid-template-columns Not supported Works in 7 of 21
height Works Works in 16 of 21
hyphenate-character Untested Works in 4 of 21
hyphenate-limit-chars Untested Works in 1 of 21
hyphens Untested Works in 4 of 21
inert Untested Works in 3 of 21
inline-size Not supported Works in 4 of 21
inset Untested Works in 3 of 21
justify-content Works Works in 13 of 21
left Works Works in 11 of 21
letter-spacing Works Works in 20 of 21
light-dark Untested Works in 3 of 21
line-height Works Works in 20 of 21
list-style Works Works in 11 of 21
list-style-image Works Works in 6 of 21
list-style-position Works Works in 19 of 21
list-style-type Works Works in 21 of 21
margin Works Works in 8 of 21
margin-block-start-end Untested Works in 4 of 21
margin-inline-block Works Works in 6 of 21
margin-inline-start-end Untested Works in 4 of 21
mask-image Untested Works in 4 of 21
max-block-size Untested Works in 5 of 21
max-height Works Works in 20 of 21
max-inline-size Untested Works in 4 of 21
max-width Works Works in 20 of 21
min-block-size Untested Works in 4 of 21
min-height Works Works in 19 of 21
min-inline-size Untested Works in 5 of 21
min-width Works Works in 20 of 21
mix-blend-mode Not supported Works in 13 of 21
modern-color Works Works in 4 of 21
nesting Untested Works in 2 of 21
object-fit Works Works in 14 of 21
object-position Works Works in 14 of 21
opacity Works Works in 19 of 21
orphans Untested Works in 10 of 21
outline Works Works in 20 of 21
outline-offset Works Works in 7 of 21
overflow Works Works in 4 of 21
overflow-wrap Untested Works in 2 of 21
padding Works Works in 20 of 21
padding-block-start-end Untested Works in 4 of 21
padding-inline-block Untested Works in 3 of 21
padding-inline-start-end Untested Works in 4 of 21
position Partial. Partial Works in 2 of 21
resize Untested Works in 1 of 21
rgb Works Works in 12 of 21
rgba Works Works in 12 of 21
right Works Works in 11 of 21
scroll-snap Not supported Works in 6 of 21
shape-margin Untested Works in 6 of 21
shape-outside Untested Works in 6 of 21
system-ui Works Works in 12 of 21
tab-size Untested Works in 8 of 21
table-layout Untested Works in 16 of 21
text-align Works Works in 13 of 21
text-align-last Untested Works in 7 of 21
text-decoration Works Works in 14 of 21
text-decoration-color Works Works in 17 of 21
text-decoration-line Untested Works in 16 of 21
text-decoration-skip-ink Works Works in 7 of 21
text-decoration-style Untested Works in 16 of 21
text-decoration-thickness Works Works in 7 of 21
text-emphasis Untested Works in 9 of 21
text-emphasis-position Untested Works in 4 of 21
text-indent Works Works in 13 of 21
text-justify Untested Works in 0 of 21
text-orientation Untested Works in 13 of 21
text-overflow Works Works in 13 of 21
text-shadow Works Works in 12 of 21
text-transform Works Works in 20 of 21
text-underline-offset Works Works in 7 of 21
text-underline-position Untested Works in 8 of 21
text-wrap Untested Works in 4 of 21
top Works Works in 11 of 21
transform Works Works in 7 of 21
transition Works Works in 4 of 21
unit-ch Works Works in 15 of 21
unit-cm Works Works in 20 of 21
unit-em Works Works in 20 of 21
unit-ex Works Works in 20 of 21
unit-in Works Works in 20 of 21
unit-initial Works Works in 12 of 21
unit-mm Works Works in 20 of 21
unit-pc Works Works in 20 of 21
unit-percent Works Works in 20 of 21
unit-pt Works Works in 20 of 21
unit-px Works Works in 20 of 21
unit-rem Works Works in 15 of 21
unit-vh Not supported Works in 17 of 21
unit-vmax Works Works in 15 of 21
unit-vmin Works Works in 15 of 21
unit-vw Not supported Works in 18 of 21
user-select Untested Works in 3 of 21
vertical-align Works Works in 21 of 21
visibility Works Works in 13 of 21
white-space Works Works in 15 of 21
white-space-collapse Untested Works in 4 of 21
widows Untested Works in 10 of 21
width Works Works in 20 of 21
word-break Works Works in 4 of 21
word-spacing Untested Works in 14 of 21
writing-mode Not supported Works in 13 of 21
z-index Works Works in 17 of 21