The env() CSS function can be used to insert the value of a user-agent defined environment variable into your CSS, in a similar fashion to the var() function and custom properties. The difference is that, as well as being user-agent defined rather than author-defined, environment variables are globally scoped to a document, whereas custom properties are scoped to the element(s) on which they are declared.... Another use case for env() variables is for desktop...agent-defined property names are case-sensitive. Formal syntax <env()>...