aria-readonly for button role

[kanggeon]

It is possible to have "toggle" style buttons with aria-pressed. Such buttons convey information much like checkboxes (which are specified for aria-readonly). Since the state of the button can provide information relevant to the user, it should be (1) focusable & (2) have the read only state announced to the user. (2) is not being met by screen readers because aria-readonly is not specified for the button role.

[jnurthen]

We don't think this is a good idea.
If you need to do this today you can use a checkbox to do it. Adding extra attributes on buttons would only lead to confusion.