Firefox 158 Makes Typed Arithmetic in calc() Baseline, So Divide Lengths by Lengths

Firefox 158 supports typed arithmetic in math functions such as calc(), which brings the feature to Baseline Newly available, alongside Chrome and Safari. In plain terms, you can divide two values of the same type, even with different units, and get a unitless number back.

That sounds small, but it solves a real problem. Fluid type and spacing systems often need a number, not a length, to drive something like opacity or line-height. Previously that meant a script or a custom property with a hand-typed number.

Now the browser can work out how far a container is along a range, and that unitless result can drive other properties. Multiply it by a unit and you can convert between types too.

Our advice is to look for places where you currently measure something in JavaScript only to turn it into a number for CSS. Fluid scales and proportional sizing are the best candidates.

Check your analytics for older browsers before you remove a fallback, but for most projects this is ready to use now.