placeholder attribute is invalid on most input types

[ffoodd]

The placeholder attribute is only allowed on email, number, password, search, tel, text and url input types, as stated by MDN and warned by the Validator. Every other case is useless.

Even though I know it doesn't seem problematic for any AT, this is sometimes pointed out in accessibility audits.

I tried several things to either only add this attribute for said input types, or the other way around dropping it when invalid—but didn't manage to make this work.

I don't think this is something to tackle in Lion, but if you have any idea on how to handle this as a subclasser to pass my current accessibility audit, I'd love some help :)