[css-scoping] Scoping of Resource Locators (url(#ID-references)) in property values

The Shadow Encapsulation section[1] doesn't mention scoping of Resource 
Locators[2], i.e. url(#id). It may not even make sense to scope url() since 
shadow content doesn't have a URL distinct from its host's, I assume, but it's a 
common authoring need to have resource locators in shadow content resolve 
against elements in the shadow content rather than the host. (For example, if 
the shadow content contains an SVG <mask id="myClipPath"> and <div 
style="mask:url(#myClipPath)">.) All the same, given that this is a common need 
and that this section is a logical place to look for the specified behavior, it 
would be good if this section either defined the behavior or had a note about 
what the intended behavior is and where it's defined.

Regarding the wider issue of how to meet the authoring need, if it doesn't make 
sense to scope url() perhaps we need a new type of resource locator distinct 
from url()?

1. https://drafts.csswg.org/css-scoping/#shadow-dom
2. https://drafts.csswg.org/css-values-3/#url-value