HTML imports and rendering

HTML imports don't seem to block the rendering of the HTML page at all.

If this is the case, custom elements must always be progressively enhanced
with a flash of content and style. This is not always ideal. It can make
sense for certain custom elements to only display after having their style
and initial scripts run.

Has there been any discussion around having attributes on the link tag to
specify blocking behaviour at all? Surely it is important to have control
over HTML rendering at this level?

Yes, build tools / multiplexing should occur for these blocking parts when
used in production for performance reasons, but I don't believe the fact
that it can become a performance issue is reason enough to force all
components to use progressive enhancement. Or am I missing something here?