font-size-adjust: Add note about scaling behavior of font family weights and styles

MDN URL

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/font-size-adjust

What information was incorrect, unhelpful, or incomplete?

I think it would be helpful to add a note clarifying that font size adjustments affect font weights, styles, and variants of the same font family.

Many font families have slightly different ex-heights for each of their different weights in the family. So a paragraph with a bolded word in it may not scale proportionally as expected with its counterparts of the same font family

What did you expect to see?

I had thought this property would scale sizes only between font families rather than also between every weight/style/variant of the same font family.

Others might find it helpful if this caveat is noted in the docs.

Do you have anything more you want to share?

Here's an example video showing what happens to a bolded word of the same font family (-apple-system) when you toggle html { font-size-adjust: ex-height from-font; } on and off in Firefox (v146.0.1)

https://github.com/user-attachments/assets/8e94daa3-7428-488d-a7c2-84a463d39305

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And here are two screenshots comparing the ex-heights of normal and bold weight for -apple-system:

Normal weight ⤵

Bold weight ⤵

MDN metadata

• Folder: en-us/web/css/reference/properties/font-size-adjust
• MDN URL: https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/font-size-adjust
• GitHub URL: https://github.com/mdn/content/blob/main/files/en-us/web/css/reference/properties/font-size-adjust/index.md
• Last commit: https://github.com/mdn/content/commit/33094d735e90b4dcae5733331b79c51fee997410
• Document last modified: 2025-12-16T15:14:26.000Z