font-size-adjust

Adjusts the size of lowercase letters relative to the size of uppercase letters.

Estimated Support About

36.59% + 17.07% = 53.66%

Apple Mail

macOS

11
12
13
1
14
15
26

iOS

11
12
13
14
15
16
17
18
26

Gmail

Desktop Webmail

2026-04

iOS

2026-04
2

Android

2026-04
2

Mobile Webmail

2026-04

Outlook

Windows

2013
2016
2019
2021
2024

Windows Mail

2026-04

macOS

2026-04

Outlook.com

2026-04

iOS

2026-04

Android

2026-04

Yahoo! Mail

Desktop Webmail

2026-04
3

iOS

2026-04
3

Android

2026-04
3

AOL

Desktop Webmail

2026-04

iOS

2026-04

Android

2026-04

Samsung Email

Android

2026-04

Mozilla Thunderbird

macOS

149

ProtonMail

Desktop Webmail

2026-04

iOS

2026-04

Android

2026-04

Fastmail

Desktop Webmail

2026-04

HEY

Desktop Webmail

2026-04

Orange

Desktop Webmail

2026-04

iOS

2026-04

Android

2026-04

LaPoste.net

Desktop Webmail

2026-04

SFR

Desktop Webmail

2026-04

iOS

2026-04

Android

2026-04

GMX

Desktop Webmail

2026-04
1

iOS

2026-04

Android

2026-04

1&1

Desktop Webmail

2026-04

Android

2026-04

Mail.ru

Desktop Webmail

2026-04

WEB.DE

Desktop Webmail

2026-04
1

iOS

2026-04

Android

2026-04

WP.pl

Desktop Webmail

2026-04

Notes

Rendering may depend on browser and OS support, Also applies to mobile client apps.

  • 1

    Partial support. Supports one numerical value, 2-value font-metric syntax unsuported (ex-height, cap-height, ch-width, ic-width, ic-height).

  • 2

    Not supported with non Google accounts.

  • 3

    2-value syntax is supported, but the first value (font-metric) is ignored.

Resources

Sponsor

Resend

Sponsored by Resend. Email for developers. The best way to reach humans instead of spam folders. Deliver transactional and marketing emails at scale.

Know something we don't?

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Test it yourself

This feature was last tested on April 16, 2026.

If you want to test this feature in the same conditions as we did, you can get our test code and run a test by yourself. Make sure to follow our testing recommendations first.

View the test code View test results on Testi@