[cssom-1] Serializing via cssText doesn't give url(<string>)

[gsnedders]

From https://bugs.webkit.org/show_bug.cgi?id=268742:

const style = document.createElement('style')
document.body.append(style)
style.sheet.insertRule(`h6 {
    background: url(/my-image.png), var(--my-image);
}`)
console.log(style.sheet.cssRules[0].cssText)

Safari outputs:

h6 { background: url(\/my-image\.png), var(--my-image); }

Chrome and Firefox outputs:

h6 { background: url(/my-image.png), var(--my-image); }

Per spec, cssText returns "a serialization of the CSS rule".

A URL is serialized as '"url(", followed by the serialization of the URL as a string, followed by ")"', which implies the above should be url("/my-image.png"), contrary to all major browsers.

[cdoublev]

If I am not mistaken, Chrome and FF serialize the original input string for values containing arbitrary substitutions like var() (cf. #6484).