Make serializeToString serialize document nodes correctly
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This is the fix for ScriptThread panic when `new XMLSerializer().serializeToString(document);` is called.
r?@jdm
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Implement DocumentFragment XML serialization.
When serializing a DocumentFragment node, we should follow [this guy](https://w3c.github.io/DOM-Parsing/#dfn-xml-serializing-a-documentfragment-node) and serialize the node's immediate children. This commit makes that change.
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Implement srcObject logic for Blob media providers
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Expose public getter to stylesheet's owner
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- [x] expose a public getter on CSSStyleSheet to return the stylesheet's owner
- [x] in CSSRuleList::insert_rule, use the new getter to pass a non-None value as an argument to arc.insert_rule
- [x] add a test for this that verifies that the new stylesheet is actually loaded
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- [x] These changes fix#23028
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In BC event-loop, only run tasks related to fully-active documents
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- [ ] These changes fix#22792 (GitHub issue number if applicable)
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