feat: Enhanced XHTML support

[corrideat]

Closes #16540
Follow up to a86e52b

This PR adds support for using Svelte in XHTML and XML documents by:

1. Producing XML-compliant output for boolean attributes (e.g., required becomes required="")
2. Using proper namespace checks for uses of tagName / nodeName, as well as ensuring that the values have the correct case.
3. Replaces use of document.createElement with document.createElementNS, defaulting to the HTML namespace. This ensures that even documents loaded in pure XML mode (e.g., index.xml, as opposed to index.xhtml) will work correctly.

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Latest commit: c11d75a

The changes in this PR will be included in the next version bump.

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pnpm add https://pkg.pr.new/svelte@17418

[7nik]

What is the point of running Svelte on XML pages? It introduces checks and uppercasing which are wasteful for 99.9999999% of users.
Generating XML pages with Svelte is ok and does not require these changes.

[corrideat]

Generating XML pages with Svelte is ok and does not require these changes.

Is it, though? It looks like it requires at least the changes to template.js (or similar changes) so that boolean attributes have a value.

It introduces checks and uppercasing which are wasteful for 99.9999999% of users.

The element.namespaceURI === NAMESPACE_HTML mostly are there for correctness and could probably (*) be removed. The point about wasteful uppercasing is valid, but it seemed like a pretty minor concern, considering that uppercasing is almost free, and that it's only done in a few places.

(*) Meaning, the check isn't necessary for the purposes of this PR the same way uppercasing is. If it's certain that elements won't have another namespace, the check is redundant.

What is the point of running Svelte on XML pages?

Is this question about documents served as text/xml or with the .xml extension (from file://), or is it about documents served as application/xhtml+xml or with the .xhtml extension? (Or possibly both?)

If it's regarding XHTML, it's IMHO a valid use-case for the framework, and one that didn't seem too hard to support. To support this, two changes were needed: the change in template.js (critical) and the uppercasing of .tagName / .nodeName (less critical but necessary).

If it's regarding XML, the answer is less clear, as probably using the correct file type would be preferred. However, again it didn't seem too hard to support (basically, just using createElementNS) and did in fact result in simplifying the code.

[7nik]

Generating XML pages with Svelte is ok and does not require these changes.

Is it, though?

Under "these" I meant the mentioned changes.

If it's regarding XML, the answer is less clear, as probably using the correct file type would be preferred. However, again it didn't seem too hard to support (basically, just using createElementNS) and did in fact result in simplifying the code.

I think the user just should set the correct file/content type.
It's funny that I found an example of XHTML page, but due to Content-Type: text/html is was rendered as HTML5 😆

Also, you should add to the code comments that those operations are needed for XHTML compliance.