CSS property · 11 real installs 14 Oct 2026 · data last changed 10 Nov 2026
font-weight in email clients
Works in 16 of 21. Partial in Outlook Classic, Yahoo Mail, Yahoo Mail Android and 2 more.
Your reader in Outlook Classic may not see font-weight the way you wrote it, even if it looks right where you tested it.
caniemail (MIT) via @emailens/engine 0.18.22
What to do instead in Outlook Classic
Design with 400 and 700, the two weights every client can draw. If a heading must be semibold where the font allows it, write 600 rather than 500, so Outlook Classic rounds it up to bold instead of down to normal. Keep weights to round hundreds, and set the weight inline on the text element so it survives a stripped <style> block.
- Partial support.
<number>values between 0 and 599 are set as normal font weight.<number>values between 600 and 1000 are set as bold font weight.
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font-weight on 11 real installs
One Emailens probe, screenshotted on each install on 14 October 2026, next to our live preview of the same client. Where font-weight is supported you should see: four distinct weights.
caniemail: font-weight is partly supported in Outlook Classic.
- Outlook 2019 (Windows) · counted as Outlook Classic Real screenshot
- Outlook 2016 (Windows) · counted as Outlook Classic Real screenshot
- Outlook 2007 (Windows) · counted as Outlook Classic Real screenshot
Partial support. <number> values between 0 and 599 are set as normal font weight. <number> values between 600 and 1000 are set as bold font weight.
caniemail: font-weight is partly supported in Yahoo Mail.
Partial support. <number> values are not supported as per CSS Fonts Level 4 where any <number> value between 1 and 1000 (inclusive) is a valid value. Only the following numeric values are supported: 100, 200, 300, 400, 500, 600, 700, 800, and 900.
caniemail: font-weight works in Outlook 365.
This Outlook.com install drops the <style> in the head, so only inline styles apply.
caniemail: font-weight works in Apple Mail.
caniemail: font-weight works in Apple Mail iOS.
Also captured, not counted as evidence (4)
- Outlook for Microsoft 365 · not mapped to an engine client yet Real screenshot
- Outlook 2019 for Mac (legacy app) · not counted as Outlook for Mac Real screenshot
This is the legacy Outlook for Mac app. The new Outlook for Mac, which the table describes, drops gradients, grid sizing functions, var() and more.
- Gmail (web) · not counted as Gmail Real screenshot
This Gmail install does not apply Gmail's sanitiser, so it shows more than real Gmail does.
- Windows Mail · not mapped to an engine client yet Real screenshot
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All 21 clients, problems first
| Client | Support |
|---|---|
| Outlook Classic | Partial See caveat 1 |
| Yahoo Mail | Partial See caveat 2 |
| Yahoo Mail Android | Partial See caveat 2 |
| Yahoo Mail iOS | Partial See caveat 2 |
| AOL Mail | Partial See caveat 2 |
Works in 16: Gmail, Gmail Android, Gmail iOS and 13 more. Show all
| Client | Support |
|---|---|
| Gmail | Works |
| Gmail Android | Works |
| Gmail iOS | Works |
| Outlook 365 | Works |
| Outlook (New) | Works |
| Outlook iOS | Works |
| Outlook Android | Works |
| Outlook for Mac | Works |
| Apple Mail | Works |
| Apple Mail iOS | Works |
| Samsung Mail | Works |
| Thunderbird | Works |
| HEY Mail | Works |
| Proton Mail | Works |
| Fastmail | Works |
| Superhuman | Works |
Caveats (2)
- 1. Outlook Classic: Partial support.
<number>values between 0 and 599 are set as normal font weight.<number>values between 600 and 1000 are set as bold font weight. - 2. Yahoo Mail, Yahoo Mail Android, Yahoo Mail iOS, AOL Mail: Partial support.
<number>values are not supported as per CSS Fonts Level 4 where any<number>value between 1 and 1000 (inclusive) is a valid value. Only the following numeric values are supported: 100, 200, 300, 400, 500, 600, 700, 800, and 900.
Caveats as recorded by caniemail (MIT), one entry per wording.
How font-weight behaves
font-weight has only two settings in Outlook Classic: any value up to 599 renders as normal and 600 or above renders as bold, so a heading set to 500 goes out looking like body text. Everywhere else the limit is the font itself, because a weight can only render if the font has a cut for it.
- Outlook Classic maps every number to one of two weights: 0 to 599 is normal and 600 to 1000 is bold. font-weight: 500 is plain text there, and 600 looks the same as 700.
- Yahoo Mail and AOL only honor the nine round values from 100 to 900. A variable-font value such as 550 falls outside that list, so use the nearest hundred.
- Web-safe fonts such as Arial and Georgia ship in regular and bold only. font-weight: 300 on Arial draws as regular and 600 draws as bold, in every client, because no other cut exists.
- A custom font only shows its in-between weights where the @font-face loads and that cut is declared. Where it does not load, the fallback font takes over and you are back to regular and bold.
Questions
Why is font-weight 500 not bold in Outlook?
Outlook Classic sets every value from 0 to 599 as normal. 500 renders as regular text. Use 600 or above if the text has to look bold there.
Does font-weight 600 work in email?
Yes, with a caveat. Outlook Classic shows it as bold, and fonts without a semibold cut, such as Arial, also draw it as bold. A true semibold appears only where the font has that cut.
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