[css-forms-1] Specify readonly input styles

[lukewarlow]

We need to add styles for readonly input styles, currently browsers seem to render them identically to read-write inputs BUT there's differing behaviour that would be good to standardise.

Chromium lets you focus the readonly input and even renders a text caret (but it doesn't blink as if caret-animation: manual was specified but it's not)

Firefox lets you focus the input but no text caret is rendered.

WebKit lets you focus but doesn't render a text caret and no focus outline ring is rendered...

I also wonder if rendering it like it's a normal input is actually the correct approach? Does it make more sense to render closer to that of a disabled input?