The template.content bridge (#463) routed every element through
content_source(), which called get_node().clone() to read the Copy
template_contents field. That deep-cloned each element's Node (its attrs
Vec plus a String per attribute) on every innerHTML/outerHTML/dump-html
serialization, once per non-void element.
template_contents is already destructured from the node in the serialize
loop, so read it there directly. content_source() (inner_html path) now
reads it through with_node() by reference. Serialization output is
byte-identical; ~10% faster on serialization-heavy pages.
SGavrl
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