"content" property is always enclosed in ""

for example:

```
content : 'attr(data-label)',
```

renders as

```
content: "attr(data-label)";
```

instead of

```
content: attr(data-label);
```

Is not easy to fix this issue...

Start by reproducing the reported conversion from the `content` property to generated CSS, comparing the quoted and unquoted outputs shown in the issue. Trace the serialization path for this property and consider the issue complete when `attr(data-label)` is emitted without enclosing quotes while ordinary string values retain their expected formatting.

Written by the indexing model from the issue text.