In 4.10.18.7.1...

"The "off" keyword indicates either that the control's input data is particularly sensitive (for example the activation code for a nuclear weapon); or that it is a value that will never be reused (for example a one-time-key for a bank login) and the user will therefore have to explicitly enter the data each time, instead of being able to rely on the UA to prefill the value for them; or that the document provides its own autocomplete mechanism and does not want the user agent to provide autocompletion values."

In 4.10.18.7.2...

"When an element's autofill field name is "off", the user agent should not remember the control's data, and should not offer past values to the user.

NOTE: In addition, when an element's autofill field name is "off", values are reset when traversing the history."

@jeffk - Ok, I now understand where you are getting this interpretation.

I think this is a dangerous interpretation (and perhaps it requires altering the spec to say must). Again Application developers need a reliable, durable way to tell the UA that a particular field should never be autofilled or autocompleted. How else do you propose we do that, other than following 4.10.18.7.1.

"If the element is mutable, its value SHOULD (emphasis mine) be editable by the user. User agents must not allow users to insert U+000A LINE FEED (LF) or U+000D CARRIAGE RETURN (CR) characters into the element's value."

https://html.spec.whatwg.org/multipage/input.html

So, according to this spec, the UA is allowed to make an input field type=text non-editable if it so chooses.

Would you argue that this is another place where Chrome would be allowed to act in a manner differently than expected, because "SHOULD" was used?