SVG elements not displaying when passed as slot content

Describe the bug

If you have an SVG component that allows you to pass in child content as a slot like so...

<svg width="150" height="50">
  <slot></slot>
</svg>

...no content will display if you pass in content wrapped in an anchor tag like so...

<script>
  import Svg from './Svg.svelte';
</script>

<Svg>
  <a href="https://example.com">
    <path d="M0,0L50,50Z" stroke="#000" stroke-width="2"></path>
  </a>
</Svg>

However, if you don't use an SVG component and simply write plain SVG, everything displays fine.

<svg width="150" height="50">
  <a href="https://example.com">
    <path d="M0,0L50,50Z" stroke="#000" stroke-width="2"></path>
  </a>
</svg>

It seems that something is breaking when the child content gets sent through the slot.

Reproduction

https://svelte.dev/repl/08270639a2154ea18192cbdf30861da8?version=3.49.0

Logs

No response

System Info

REPL

Severity

blocking all usage of svelte

Very similar to https://github.com/sveltejs/svelte/issues/7450 and https://github.com/sveltejs/svelte/issues/7563 (basically a duplicate I would say)

Array.from(document.querySelectorAll('a')).map(e => e.namespaceURI)

0: "http://www.w3.org/2000/svg" <=============
1: "http://www.w3.org/1999/xhtml" 

If you can't add <svelte:options namespace="svg"> into all nested components you can use this action :

<script lang="ts">

  /** Ensure all "a" elements inside an SVG node belong to the correct namespace */
  function ensureSVGA(node: SVGSVGElement) {
    const namespaceSVG = 'http://www.w3.org/2000/svg'
    const links = node.querySelectorAll<HTMLLinkElement>('a')
    for (const link of links) {
      if (link.namespaceURI === namespaceSVG) continue
      const a = document.createElementNS(namespaceSVG, 'a')
      for (const { name, value } of link.attributes) {
        a.setAttribute(name, value)
      }
      a.append(...link.children)
      link.insertAdjacentElement('beforebegin', a)
      link.remove()
    }
  }
</script>

<svg use:ensureSVGA>
  <slot />
</svg>

I've found a similar problem when loading SVG defs via a svelte:fragment when the svg element is also a slot and the parent element is an HTML div.

Here's the repro: https://svelte.dev/repl/2647caa253d648428dfd96b8d1d6b974?version=3.55.0

If you comment out the main wrapper div, svelte is able to detect the SVG namspace and the gradient shows up. If you wrap the whole thing in a div, it will interpret <linearGradient> an HTML tag. I tried adding <svelte:options namespace="svg"/> to the Svg.svelte component but that has no effect.

@mhkeller can you <svelte:options namespace="svg"/> in nested component? https://svelte.dev/repl/822371299ddf401885091c66715c66f7?version=3.55.0

Yes that it as a viable workaround for now – similar to adding the xmlns attribute inline to the linearGradient tag. It would still be nice if Svelte detected that the children of the defs object were svg elements, though.