Nested HTML element not assigning to slot

Question

In the below code I am trying to assign <span slot='test-slot'>b</span> to <slot name='test-slot'>a</slot> but the assignment does not work. If I bring <span slot='test-slot'>b</span> outside of its parent <div> container the assignment does take place as expected.

Why is this? Is there anyway you can assign from nested elements with the slot element? If not, any alternatives? This is obviously a reduced test case but in my real web component, it is much more intuitive for a user to add an element with the slot tag within other containers.

<test-element>
  <div>
    <span slot='test-slot'>b</span>
  </div>
</test-element>

<template id='template-test-element'>
    <slot name='test-slot'>non slotted content</slot>
</template>

<script>
  class TestElement extends HTMLElement {
    constructor() {
      let template = document.getElementById("template-test-element")
                             .content.cloneNode(true);
      // Initialise shadow root and attach table template
      super() // sets AND return 'this' scope
        .attachShadow({mode:"open"}) // sets AND returns shadowRoot
        .append(template);
    }
  }

  customElements.define('test-element', TestElement);
</script>

Answer 1

Named slots only accept top-level children that have a matching slot-attribute.

See this (old) Polymer explainer or this more recent article.

Edit: Not sure where this is coming from though, the spec fails to mention this requirement: https://html.spec.whatwg.org/multipage/dom.html#attr-slot

It also is neither mentioned here nor here.