https://drafts.csswg.org/css-values-3/#url-empty, with tests at https://github.com/web-platform-tests/wpt/blob/master/css/css-values/urls/empty.html
This section states:
If the value of the url() is the empty string (like url("") or url()), the url must resolve to an invalid resource (similar to what the url about:invalid does).
This doesn't appear to define what the computed value of an empty url() is, thus it would appear to be undefined. The tests (by @emilio) assert that the value is about:invalid, rather than it merely being similar to it (resolving to an abstract invalid resource).
Compare with, under Relative URLs above:
When a
<url>appears in the computed value of a property, it is resolved to an absolute URL, as described in the preceding paragraph. The computed value of a URL that the UA cannot resolve to an absolute URL is the specified value.
Current implementation status is:
about:invalidRetroSearch is an open source project built by @garambo | Open a GitHub Issue
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HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.3