In human resources employment you can always do business casual for example: women can wear a skirt that doesn't go above her knee or pants. Something I see that are not allowed most of the time with any hr jobs would be those short pants. Any pant or skirt that comes above the knee. Women have all kinds of nice shirts that are appropriate. Just make sure it hides all of your assets with no cleavage showing. Women also need to make sure that extra under garments are not able to be seen when sitting, standing, or doing anything else. That is very inappropriate.
Now men that hold human resources job have few choices. Men could wear slacks/pants with a dress or button down collared shirt. Usually no sneakers or tennis shoes are allowed. With men now you have to make sure that your undergarments are not showing when doing for any work. Although human resources job usually do not require any physical work.
There are many who decide not to follow guidelines when entering into an interview. And just because the way they were dressed they did not get the job. I have seen where men come in still with their pants below their hips and the interviewer has to ask for you to pull up your pants. With women I have seen where they come in dressed with skin tight pants on a shirt that does not cover any of their midriffs. This is not ok. Most HR careers will look at you in disgust and it doesn't matter what type of experience you have.
As always, a rule of thumb follow strict guidelines when going for an interview with human resources employment it will get you further.