CSS Properties

box-sizing

CSS property · 15 supported, 0 partial, 6 unsupported across 21 clients

Client Support

Client	Category	Engine	Support
Gmail	webmail	Gmail Web	Supported
Gmail Android	mobile	Gmail Mobile	Supported
Gmail iOS	mobile	Gmail Mobile	Supported
Outlook 365	webmail	Outlook Web	Supported
Outlook (New)	desktop	Outlook Web	Unsupported
Outlook Classic	desktop	Microsoft Word	Unsupported
Outlook iOS	mobile	Outlook Mobile	Supported
Outlook Android	mobile	Outlook Mobile	Supported
Outlook for Mac	desktop	WebKit	Supported
Apple Mail	desktop	WebKit	Supported
Apple Mail iOS	mobile	WebKit	Supported
Yahoo Mail	webmail	Yahoo	Unsupported
Yahoo Mail Android	mobile	Yahoo	Unsupported
Yahoo Mail iOS	mobile	Yahoo	Unsupported
Samsung Mail	mobile	Samsung	Supported
Thunderbird	desktop	Gecko	Supported
HEY Mail	webmail	WebKit	Supported
Proton Mail	webmail	Proton	Supported
AOL Mail	webmail	AOL	Unsupported
Fastmail	webmail	Fastmail	Supported
Superhuman	desktop	Blink	Supported

Client-by-client behaviour for box-sizing

Fully supports box-sizing (15): Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Proton Mail, Fastmail, Superhuman.

No support (6): Outlook (New), Outlook Classic, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail. Plan fallbacks for these clients before relying on box-sizing in production sends.

Fixes & Workarounds

Outlook (New)

Unsupported

Account for padding in your width calculations (use padding on a nested element).

Before

<div style="width: 300px; padding: 20px;
            box-sizing: border-box;">
  Content — total width stays 300px
</div>

After

<!-- Set width to content-width (300 - 40 = 260px) -->
<div style="width: 300px;">
  <div style="padding: 20px;">
    Content — padding on inner element
  </div>
</div>

Account for padding in width manually (no box-sizing)

Outlook Classic

Unsupported

Account for padding in your width calculations (use padding on a nested element).

Before

<div style="width: 300px; padding: 20px;
            box-sizing: border-box;">
  Content — total width stays 300px
</div>

After

<!-- Set width to content-width (300 - 40 = 260px) -->
<div style="width: 300px;">
  <div style="padding: 20px;">
    Content — padding on inner element
  </div>
</div>

Account for padding in width manually (no box-sizing)

Yahoo Mail

Unsupported

Account for padding in your width calculations (use padding on a nested element).

Before

<div style="width: 300px; padding: 20px;
            box-sizing: border-box;">
  Content — total width stays 300px
</div>

After

<!-- Set width to content-width (300 - 40 = 260px) -->
<div style="width: 300px;">
  <div style="padding: 20px;">
    Content — padding on inner element
  </div>
</div>

Account for padding in width manually (no box-sizing)

Yahoo Mail Android

Unsupported

Account for padding in your width calculations (use padding on a nested element).

Before

<div style="width: 300px; padding: 20px;
            box-sizing: border-box;">
  Content — total width stays 300px
</div>

After

<!-- Set width to content-width (300 - 40 = 260px) -->
<div style="width: 300px;">
  <div style="padding: 20px;">
    Content — padding on inner element
  </div>
</div>

Account for padding in width manually (no box-sizing)

Yahoo Mail iOS

Unsupported

Account for padding in your width calculations (use padding on a nested element).

Before

<div style="width: 300px; padding: 20px;
            box-sizing: border-box;">
  Content — total width stays 300px
</div>

After

<!-- Set width to content-width (300 - 40 = 260px) -->
<div style="width: 300px;">
  <div style="padding: 20px;">
    Content — padding on inner element
  </div>
</div>

Account for padding in width manually (no box-sizing)

AOL Mail

Unsupported

Account for padding in your width calculations (use padding on a nested element).

Before

<div style="width: 300px; padding: 20px;
            box-sizing: border-box;">
  Content — total width stays 300px
</div>

After

<!-- Set width to content-width (300 - 40 = 260px) -->
<div style="width: 300px;">
  <div style="padding: 20px;">
    Content — padding on inner element
  </div>
</div>

Account for padding in width manually (no box-sizing)

Production guidance for box-sizing

box-sizing is a CSS property. Across the email client matrix, 15 of 15 tracked clients support it fully, 0 only partially, and 6 not at all. The clients most likely to require a workaround for this feature are Outlook (New), Outlook Classic, Yahoo Mail. Each of them needs a tailored fallback. See the per-client examples above for the specific code fix Emailens recommends.

When in doubt, ship a baseline that renders without box-sizing and treat it as progressive enhancement on clients where support is verified. Emailens automatically detects cross-client divergence on this feature when you preview an email, so you can see exactly which template paths break before a campaign goes out.

Related Features

!important accent-color align-items animation aspect-ratio backdrop-filter background background-blend-mode

Test your email across all 21 clients

Preview rendering, catch CSS issues, and get actionable fixes. Free.

Try Emailens Free

Support data last updated Apr 27, 2026 · synced from caniemail.com via @emailens/engine.