Define when to fetch subresources (icons and shortcuts) + which client to use

[marcoscaceres]

See:
https://html.spec.whatwg.org/multipage/infrastructure.html#critical-subresources

Applies to shortcuts and icons.

[aarongustafson]

Make a note about these being critical & needing to be downloaded during processing.

[marcoscaceres]

We need to integrate into the processing algorithm, after processing, they image resources should be downloaded.

[aarongustafson]

We talked about using CSS as an example… progress with this has been slow over there: w3c/csswg-drafts#1088

[aarongustafson]

@marcoscaceres Per whatwg/html#5581 (comment), it seems like we may not need to do this task right now, no?

[marcoscaceres]

Correct, but we do need to do it when processing happens so definitely as part of CR... but happy to wait until scope and start URL are all sorted.