XHTML - disable automatic browser spellcheck on textareas

I have a textarea with random base64 code in it. The text inside it has a annoying thin red line underlining it in Opera.

Is there a opera html attribute that can disable the spell checker on that field?

spellcheck="false" seems to work in Chrome, but not in Opera

Have you tried making the textarea readonly by adding

readonly="readonly"

? Or does the BASE64 code need to be editable?