Description
https://drafts.csswg.org/css-values-3/#local-urls
Should properties that take <url>s as images serialize fragment URLs as just the fragment or as the full, resolved URL? The spec doesn't make any distinction between <url>s for images and non-images (such as various SVG properties like marker-start). The fragment URL concept was introduced to make it easier to use local references for non-image properties when the base value might change, and local URLs don't really make sense for images.
Test: https://mcc.id.au/2018/10/url-fragment-serialize.html
Results
| background-image | mask-image | filter | marker-start | |||||
|---|---|---|---|---|---|---|---|---|
| specified | computed | specified | computed | specified | computed | specified | computed | |
| Chrome | url("#x") | url("https://mcc.id.au/2018/10/url-fragment-serialize.html#x") | url("#x") | url("#x") | url("#x") | url("#x") | ||
| Edge | url("#x") | url(https://mcc.id.au/2018/10/url-fragment-serialize.html#x) | url(#x) | none | url("#x") | url("#x") | ||
| Firefox | url("#x") | url("https://mcc.id.au/2018/10/url-fragment-serialize.html#x") | url("#x") | url("https://mcc.id.au/2018/10/url-fragment-serialize.html#x") | url("#x") | url("#x") | url("#x") | url("#x") |
| Safari | url("https://mcc.id.au/2018/10/url-fragment-serialize.html#x") | url("https://mcc.id.au/2018/10/url-fragment-serialize.html#x") | url("#x") | url("#x") | url("#x") | url("x") | ||
All four browsers I tested serialize the computed value of background-image with a fragment as the entire resolved URL. (Safari is an odd one out in serializing the entire resolved URL for the specified value, too. And there's a bug in the marker-start computed value serialization.)
I think it makes sense to treat image and non-image <url> values the same way wrt fragment URLs, but since nobody implements this behavior I wanted to check to make sure it's what we all agree with before changing.