CSS attr() concatenation with url path [duplicate]

Answer 1

It is not possible to create a composite url() value out of two or more strings. On top of the legacy url() value, which isn't even a proper CSS function (see Is there a way to interpolate CSS variables with url()? — which means you can't even do this with custom properties), the proper CSS function version of url() as defined in css-values-3 only accepts a single string.¹

You can concatenate multiple strings in a content declaration, but that is a feature of the content property, not of strings in CSS.

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¹ _{Since url() accepts a single string, this does mean that a single attr() can be used as a URL value, also new to css-values-3, as attr(image url)... except browser support is nonexistent.}