Trying to build Web Components for SVG should produce a warning/error

[akauppi]

Web Components are HTML only and not supported in the SVG namespace.

• Do Custom Elements / WebComponents work with SVG? (StackOverflow)
• Proposal: Allow custom elements to be in any namespace (WICG/webcomponents)

However, users such as me might not know this, and it would be helpful if the Svelte compiler told us so.

Describe the bug
Currently (Dec 2020), Svelte compiles a source with <svelte:options namespace="svg" tag="svsv-draggable" /> quietly.

The output contains:

SvelteElement = class extends HTMLElement {

This is enough to convince me that the code tries to make an HTML Web Component, but I haven't fully traced the output.

Expected behavior
Give the user a nice message, saying what we're trying to reach is not currently possible.

Information about your Svelte project:

• Svelte version: 3.31.0
• Whether your project uses Webpack or Rollup: Rollup

Severity
Not severe. Helpful.

Additional context
Add any other context about the problem here.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.