How many email clients support box-sizing?

It works in 16 of 21, not supported in 5.

Does box-sizing work in Apple Mail?

box-sizing works in Apple Mail. Closest to Fastmail: Apple Mail, which agrees on 192 of the 235 features both are tested on.

box-sizing in Fastmail

Works box-sizing works in Fastmail.

Works in 16 of 21 · not supported in 5

Which clients

Works: Gmail, Gmail Android, Gmail iOS, Outlook 365, Outlook (New), Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Apple Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Proton Mail, Fastmail, Superhuman
Not supported: Outlook Classic, Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail

The fix for the clients that need it

Needed in 5 of 21: Outlook Classic, Yahoo Mail, Yahoo Mail Android and 2 more.

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

instead

5 lines

<div style="width: 300px;">
  <div style="padding: 20px;">
    Content; padding on inner element
  </div>
</div>

Show the original

before

4 lines

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

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Every feature in Fastmail · box-sizing in every client

Questions

How many email clients support box-sizing?: It works in 16 of 21, not supported in 5.
Does box-sizing work in Apple Mail?: box-sizing works in Apple Mail. Closest to Fastmail: Apple Mail, which agrees on 192 of the 235 features both are tested on.

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Real captures 14 Oct 2026 on 11 installs · caniemail (MIT) via @emailens/engine 0.18.22 · tested upstream 28 Feb 2019 · data last changed 10 Nov 2026 · Outlook (New) cells are the engine's mapping