What
The wording for what an artifact can and cannot do now lists handing the viewer a file to save among the things static HTML cannot provide, and the error you get when publishing an unservable supporting file explains what supporting files are for.
Details
- The rejection now spells out that neither a served file nor a
data:/blob:link gives the viewer a file, because downloads started by the page are blocked. - Applies to the runtime-capability description and the prototype/template guidance.
Evidence
Supporting files are assets the page itself loads
Strings lifted out of the shipped bundle, so the claim above can be checked against them.