It seems using ( and " inside of a style value is not allowed, making setting background images impossible. It would be nice to be able to use custom resources for backgrounds of elements.
The expected syntax for setting a background image is the following: background-image: url("/path/to/resource.jpg")
Using the following code as the style section of a span (with debugMode set to true) it gives an error of "Failure: The style values 'url('/site/Shared Documents/imageName.jpg')' contains one or more of the following disallowed characters ( : & ; ! ."
"style": {
"display": "inline-block",
"width": "8px",
"height": "8px",
"background-image": {
"operator": "?",
"operands": [
{
"operator": "==",
"operands": [
"@currentField",
"expectedValue"
]
},
"url('/site/Shared Documents/imageName.jpg')",
""
]
}
}
This is correct. Currently style properties cannot have any of the following characters: (, :, &, ;, !
However, there is one exception and that is the use of the rgba css function:

Are there any workarounds for this? I'd like to be able to use linear-gradient for background color to show a color trend.
This definitely seems strange. Why accept a 'background-image' declaration without allowing the url for the image? Why even have it?
Is there any update on this or any more information? It's in the specs that "background-image" is allowed, but what's the point if you can't do it because ( ) isn't allowed in style? It's especially weird that rgba() background colors are allowed.
@furler This is not a supported usage. We will update the specs. Thanks for reporting this.
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This definitely seems strange. Why accept a 'background-image' declaration without allowing the url for the image? Why even have it?