I can use XML namespaces on CSS attribute selectors fine, even with pseudoclasses:

<root xmlns="http://www.tei-c.org/ns/1.0" xmlns:xml="http://www.w3.org/XML/1998/namespace">
    <s xml:id="s382.11.1" n="someValue">Content goes here</s>
</root>

@namespace url(https://www.tei-c.org/ns/1.0);
@namespace xml url(https://www.w3.org/XML/1998/namespace);
[xml|id]::before {
    content: attr(n);
}

But when applying the same namespace syntax to the attr function in a content declaration, Google Chrome 126 and Opera 106 don’t get it, although Firefox 128 does:

@namespace url(https://www.tei-c.org/ns/1.0);
@namespace xml url(https://www.w3.org/XML/1998/namespace);

s {
    display: block;
    margin: 8px 16px;
    clear: both;
}

s::before {
    content: attr(xml|id);
    display: block;
    float:left;
    width: 144px;
}

Chrome and Opera don’t display the ::before pseudoelement at all, i.e. don’t reserve the specified width either, unless I set it to a default-namespace attribute @n, in which case they behave as intended. Firefox works as intended with either attribute selected. The W3C CSS Validation Service throws a parse error at precisely this point: xml|id) (sic). It is happy if I write attr(xml:id) instead, but then no browser renders the pseudoelement. Is there some other way I can render this value? Moving the attribute to the default namespace in my XML corpus is not an option.