modulepreload is a new link relation that behaves similar to preload, but different in several key ways. For example, it interprets its attributes and changes to them differently; it uses the module map instead of the preload cache; and it allows optionally fetching descendant modules as an optimization. Spec: https://html.spec.whatwg.org/multipage/links.html#link-type-modulepreload Tests: https://github.com/w3c/web-platform-tests/blob/master/preload/modulepreload.html
FWIW the processing module for modulepreload is unrelated to that for preload so I don't think bug 1394778 is actually a dependency.
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