whatever your comfortable wearing, jeans and t-shirt if that's what you normally wear.

The #1 superhero?!? Anything he likes because he can.

But is there a visa available for mild-mannered janitors?

Clearly I can't go in my PJs as they'll unmask me at security and then where will we be?
I'll be off duty from work so won't be in my janitor garb!

I have it under control now.

Do you know whether or not he got his visa?

Many immigration lawyers recommend dressing as you would for a job interview, or a step below that. So much of the interview is subjective that you don't want to give them an excuse to treat you in a casual 'wtf' manner. "Misspelled your name, wtf--denied. Wrong date on page A6 subclause 3, wtf--denied".

I'd go with the step below it - i.e. a pair of slacks and collared shirt.

I don't think it matters to much, but these points raised by Penguinsix are valid.

Anything you would wear if you were on a first date or going out for a special evening is probably ok.

Things to not wear:
Tuxedo
Scuba gear
Red clown nose
A "T" shirt with "I Hate America" written on it
A Cuban flag hat, or any hat with red in it (they're still sensitive)
Harry Potter mask
Dixie-Chicks "T" shirt
Mr. Spock ears
Fake Hobbit feet
Nothing
A Rod Stewart wig
A Bruce Forsythe wig
and finally,
Do NOT, repeat NOT arrive dressed as one of the TellyTubbies.

i wore casual stuff for mine. Just take an umbrella as the first part of queueing is outside and if it rains you will get wet as you can be there a while.