Re: [w3c/manifest] Abstracting selecting an icon. (#710)

@marcoscaceres:

> That should "throw" a TypeError... but throwing doesn't mean anything in our format.

Hmm, yeah that's the one thing that bugs me. We can write a top-level rule that says "if the ECMAScript to IDL conversion throws a TypeError, the manifest is invalid", so it isn't meaningless that the conversion throws a TypeError outside of an actual JavaScript execution context. BUT the problem is that we don't want to trash the whole manifest if one member is invalid. If one member throws a TypeError, we generally want to ignore that member, not the whole thing.

It seems that this isn't an issue with using IDL in a document format, but rather that IDL's conversion algorithm doesn't have a "non-strict mode" (that I know of). If there was a way to say "convert an ECMAScript object to IDL, with any TypeErrors in a dictionary member causing that member to be set as undefined". Or thereabouts. That would be perfectly usable in this spec.

I also wanted to say I'm not sure it's worth breaking the current icon selection language out into its own spec. At the moment, it's just too vague to be useful. The only normative text is:

> If there are multiple equally appropriate icons in icons, a user agent MUST use the last one declared in order at the time that the user agent collected the list of icons. If the user agent tries to use an icon but that icon is determined, upon closer examination, to in fact be inappropriate (e.g. because its content type is unsupported), then the user agent MUST try the next-most-appropriate icon as determined by examining the ImageResource's members.

"appropriate" is never defined. The only real normative requirement here is that ties (of an unspecified evaluation function) are broken by which comes last. Doesn't seem worth trying to make this weak requirement available across specs. Though perhaps this could be an opportunity to better define "appropriate" icon selection.

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