[portal tag] proposal to enable portal of element / text

[yinonov]

A common practice to quote in the web is utilizing the blockquote and cite tags to present context as a quoted source.

When authoring articles, documentation in particular, there's often a need to refer a proxy or mutual member to a source from sub sections.

Best experience for a reader would be to read the context on the spot while knowing it is a portal to a different existing live source.

Let's say we document a popup element class that is integrated within a menu (menu uses popup under its hood).
when authoring a doc for each will first describe the source (popup)

<h1>Popup</h1>

<h2>Open</h2>

<p>use the `open` attribute to set the popup open state.</p>

<ul>
  <li>Type: boolean</li>
  <li>Default: false</li>
</ul>

then in menu we will need to either:

• reference - link to the popup page source. tedious for user and breaks the reading flow

<h1>Popup</h1>

<h2>Open</h2>

<p>Delegates to the [open property in the underlying popup element](https://vivid.deno.dev/popup#open).</p>

<ul>
  <li>Type: boolean</li>
  <li>Default: false</li>
</ul>

• hardcode - repeat the same description (copy & paste). Requires maintenance and can be forgotten over time

<h1>Menu</h1>

<h2>Open</h2>

<p>use the `open` attribute to set the popup open state.</p>

<ul>
  <li>Type: boolean</li>
  <li>Default: false</li>
</ul>

A better experience for a reader would be to reflect the fact that the sub section is referring to a source
GitHub are already featuring such a behavior by embedding permalinks

html/README.md

Line 5 in 823a14b

We are committed to providing a friendly, safe and welcoming environment for all. Please read and respect the [WHATWG Code of Conduct](https://whatwg.org/code-of-conduct).

we should be able to portal a window of specific area of html or text

[yinonov]

apparently this is also addressed at #6507