CSS attr() concatenation with url path [duplicate]

Question

How can I concatenate the CSS attr() selector with static text in a url() field?

The HTML I use:

<div image='/Require/static.png'></div> //Example 2
<div image='static.png'></div> //Example 3, 4, 5

For example:

//image attribute contains the image name (and prefix location when needed, see example 2)
div[image]:before {
    background-image: url('/Image/static.png'); //Works
    background-image: url(attr(image)); // Works
    background-image: url('/Image/' attr(image)); //Fails
    background-image: url('/Image/' #attr(image)); //Fails
    background-image: url('/Image/' {attr(image)); //Fails
    }

So - if it is possible - How can I achieve this?

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.}