Function serialize_html_fragment 

pub(crate) fn serialize_html_fragment<S: Serializer>(
    cx: &mut JSContext,
    node: &Node,
    serializer: &mut S,
    traversal_scope: TraversalScope,
    serialize_shadow_roots: bool,
    shadow_roots: Vec<DomRoot<ShadowRoot>>,
) -> Result<()>

https://html.spec.whatwg.org/multipage/#html-fragment-serialisation-algorithm