Switching from "visual" to "HTML" editor in WordPress

Whenever I edit a post or a page in WordPress, the “HTML” tab is greyed out – but I can’t find a setting anywhere to enable it.

Does anybody know why this tab is greyed out and how to activate it?

Maybe it’s a problem of your browser or some plugin.

Thanks for the tip – it turned out to be that in my theme, I was trying to have WordPress use the jQuery from Google’s content delivery network instead of the local jQuery file.

I guess I wasn’t doing it right, because although it worked OK in the public site, it wasn’t working in the administrative area. I ended up using the “Use Google Libraries” plugin, and that seems to work well.