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This is "relative color" syntax, it works with a range of color spaces/color functions. The key is the "from" at the front. Here's the MDN documentation: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_colors/...

It may be confusing, but everything here is static param. The —- prefix is css variables, where inside a css declaration block you write: —bg: blue

The `l` isn't!

Some newer ones like calc-size are also like this.