Import maps cannot be used with HTTP Link rel=modulepreload

[allstarschh]

When the modulepreload feature is served from HTTP Link header,
it will fetch a modulepreload module script graph, which will in turn disallow further import maps, that will make import maps won't be parsed at all, see Prepare the script element, step 32.

Latest HTML spec commit: d67f8f7

This will make import maps totally useless if the modulepreload is served from HTTP Link header.

[annevk]

cc @domenic @noamr

[domenic]

Yep, this is a known issue, and "working as intended".

Investigations into fixing this generally take the form of finding a solution for allowing import maps to update over time. See WICG/import-maps#92, https://github.com/guybedford/import-maps-extensions#lazy-loading-of-import-maps, and guybedford/import-maps-extensions#17 for the latest discussions on how we could do that. We know it's a high-priority request from web developers, but Chromium at least hasn't been able to find the engineering bandwidth for working on it so far.

/cc @hiroshige-g