Note on reply disappearing: I think sometimes HN hides the reply unless you go to the permalink (i.e. what the "2 minutes ago" link points to). Or at least that's how I work around it when I don't see the reply button. Not really sure why it does that, never looked into it.

It's not so much about the HID class itself as "why" (it's more an effect you find out later once you know what the HID class is) as it is "if an OS driver doesn't already hold the USB keyboard how could you type on it". The opening paragraph of the standard tips this off straight away:

"...Standard device classes include keyboard, mice, audio, video and storage devices. Operating systems support such devices using the "class driver" provided by the OS vendor..."

That combined with the closing paragraph of 3.1 under security considerations I quoted before led to the answer for the particular question without the explicit mention of "HID" in the document:

"Lastly, since USB devices are unable to distinguish requests from multiple sources, operating systems only allow a USB interface to have a single owning user-space or kernel-space driver. The UA acts as a user-space driver, therefore allowing only a single execution context to claim a USB interface at a time. The claimInterface() function will fail if multiple execution contexts attempt to claim an interface."

I later ran across "HID" reading a blog post while testing some code which is how I fumbled to finding WebHID. For the WebHID portion the hint I originally mentioned came from this paragraph in 3.1:

"In some cases, a device may expose functionality that should not be accessible at all from a web page. Specific devices may be blocked by recognizing the device by its vendor and product ID and hiding such devices during enumeration. The HID report descriptor allows a device to describe its own capabilities, and this information can be used to block classes of devices even when the vendor and product ID cannot be known in advance. This can be accomplished by inspecting the usage values assigned to collections within the report descriptor; for instance, access to keyboard-like devices can be denied by denying access to devices that include a top-level collection with the Keyboard usage ID."

I guess "mitigates" wasn't fair as the feature still hasn't even had an experimental implementation added behind a flag in Chrome yet let alone have a demonstrable implementation of the proposed protection. I'd say "considered" was fair though and not at all hidden/hard to find.

I also wanted to point out that the wording around "manufacturers should do" in both is presented an argument for defense in depth (IIRC they even called out defense in depth explicit in one of the calls to action) not their argument for "since manufacturers should do this we shouldn't do anything". Again about reading to find "why" vs "why not" which usually requires reading in context instead of searching for a key phrase.

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Don't get me wrong, questions are a fine method of learning! It's just your initial question was "Please point out if this attack is not viable" not an ask of "I saw this in the standard, can you explain why this attack is not viable?" so I told you it's not viable and then mentioned it's in the first 3 sections in case you were curious. The response I got back was that you had already read section 3 and got a different takeaway. Well of course you did, I didn't say why was in section 3 I said it was in the first 3 sections, which are quicker reads than the comment chain the question came from! Going on good faith I pulled the exact quote out, explained it further, and then mentioned "I thought it might work this way, show me the line that says it isn't" isn't how I want to talk about the standard to then get told my original comment which answered exactly what you asked was low effort! Looking back I can see the confusion with section 3/first 3 sections and that originally you very likely meant "could you explain why" not a literal "is it or not" so my apologies for misinterpreting where you were trying to drive the conversation but I ask you consider the above as well before judging my reply/cautiousness to be just low effort/dismissive.

Anyways I hope the content above the "." gets us on the same page. I'd have shared some modified WICG/webusb GitHub repo linked demos I worked on earlier to validate my understanding before I said "nope" but they don't work on JSFiddle/CodePen because WebUSB requires a feature-policy header to be set to be used in a page + "allow" to an iframe (which seems like other great protections but absolute pains for testing!) so you'll just have to take my word on the real world test :).