I've had some problems typing into text fields in the HTML5 palette, which has been a bit frustrating when trying to enter URLs. But I've just found the way to work that seems to completely avoid this entirely...
Simply select whatever's in there already and type something over the top or just backspace over it. This seems to make the text field get 'properly' focused, so you can then just paste or type without concern. If you need to edit something that's already in a field, select one character and type the same thing to replace it, then do your new edits.
Obviously this is something that I hope gets nailed in a bugfix round soon – new users shouldn't have to find these things out the hard way – but it's good to know there's a simple solution.