Using the Font Style rule will cause a Warning.

[neopolitans]

This error is known and is partially outside of the control of the developer* as of 2021.3.6f1 -> 2023.2

Error was caused by documentation inconsistency between Unity's USS Common Properties and Mozilla Developer Network's CSS font size pages. The .NET Library ExCSS caught the error (as seen in the image below) and provided a descriptor.

This cannot be addressed at this time for a few factors:

• USS Documentation has some errors and missing documentation between the 2021 and 2023 manual.
• CSS Documentation and how much it applies varies. Unity modified (in some ways) the version of ExCSS being used and as a result, trying to change the accepted parameter type <number> / "Number" to a Font-relative unit derivation of <Length> \ "Length" could cause unpredictable results.
• CSS states the property needs a rem, ex or em property for element-based scaling or a percentage-length value. USS/ExCSS states it needs a length (px or %) value.

Referenced Documentation: https://docs.unity3d.com/2023.2/Documentation/Manual/UIE-USS-SupportedProperties.html https://developer.mozilla.org/en-US/docs/Web/CSS/font-size

^{* Out of control to an extent.}

[neopolitans]

A temporary fix has been applied which provides an alternative constructor with a <length> parameter and marks the <number> based constructor as deprecated.