[css-forms-1] Base appearance for image inputs

[lukewarlow]

While <input type="image"> doesn't get all the styles associated with buttons, it does still get UA styles applied to it, we need to decide what we want to do for base appearance mode.

Something all browser seem to have in common is the usage of a pointer cursor. Chromium and Firefox make this explicit in the stylesheet, WebKit uses "cursor: auto" but it renders a pointer still.

Do we want to keep this? Is there a reason they have a pointer given this historically has been associated with links (though many sites expand it to cover many interactive elements nowadays).

Browsers also then do stuff such as setting initial values for stuff like background-color, so we should come up with an agreed set of styles for those too.

[lukewarlow]

For the record I know this is a rather esoteric input type and would just as happily make them display:none in base appearance mode but we should agree on something so browsers have the same computed styles.