Hello,

The W3C validator is quite outdated and is only checking for CSS3 properties. Since then, linear-gradient has been updated to support single values.

background-image: linear-gradient(hsl(0 0% 100% / .1) 0% 100%);

That code simply says, create a gradient which is hsl(0 0% 100% / .1) in color, starting from 0% and ending at 100%

The code is valid in all browsers and works as expected. In fact, MDN even states that a single value is valid, such as:

background-image: linear-gradient(red);

So technically, the code contains too many, not too few 😅

Changed Status to Not a bug