WPT html/semantics/document-metadata/interactions-of-styling-and-scripting/conditionally-block-rendering-on-link-media-attr.html sees a still-loading stylesheet as a member of document.stylesheets with empty cssRules; WPT test isn't expecting this

[pshaughn]

The test doesn't have any explicit assertions about this, it just expects that if it's looking in document.stylesheets at a stylesheet that's part of the test, .cssRules[0] on that stylesheet will be a css rule and not undefined. Possibly stylesheets are only supposed to be added to document.stylesheets after they're done loading? whatwg/html#696 suggests this might be a fine point that isn't properly specified yet.