Under U.S. Code › Title 4 › Chapter 1 › § 8, the act of stomping on an American flag would fall under at least two provisions of the code. As examples:

1. To stomp on it implies it is on the ground, which is prohibited.
2. The flag should never be disrespected.
3. The flag should not be left in a horizontal, flat position, but should fly free.

The illegality portion of your question is more difficult to answer. This Code has little in the way of enforcement. If you do bad acts toward a flag, plenty of people will be "up in arms" over your actions! Military and Vets would be very vocal about your disrespect. Yet, I've never heard of anyone being arrested for mistreatment of the US Flag. Perhaps we need to arrest, so people start to understand that the flag has meaning and value. It isn't just colored fabric.

However, I can think of one instance where others might hail an "offender" who sullies the flag. If all I had was a large flag and an injured patient I needed to move to safety, but a patient I could not carry, I'd probably put the person on the flag to move them, especially if transporting by another way could lead to more injury. I would use strips to bind the person's wounds. Now, if I was in a war zone and in desperate circumstances, I might be applauded for using this iconic symbol to save a soldier's life, the life he laid down for our flag and country. However, my actions would put the flag under every disrespect listed: laid flat, deliberately used to "carry" (something) someone, dragged across the ground, deliberately soiled or even muddied, deliberately torn apart to wear almost as clothing (bandages) , damaged, soiled and bled upon, and so it could never be displayed again as a flag. Now, would my actions be hailed for attempting to save a life? Or would I be criticized and maligned for defacing and disrespecting the flag? Such are quandaries humans must sort through to decide on the most ethical actions---even if it disobeys a Law or Code.

Quotation: The American flag represents all of us and all the values we hold sacred. - by Adrian Cronauer

The US Flag should represent these values:

1. the sacrifices that men and women made on behalf of us so we could have our freedom and liberties
   
2. color blue - the valor of our ancestors who fought and died in many battles and to our soldiers who continue to defend our flag
3. white color - for the purity in all of our hearts and the honor we should show in our everyday lives
4. red color - represents the blood shed and the millions of lives lost in service to our country, including everyday citizens
5. our individual and collective freedoms
6. as a symbol of hope to those oppressed by tyranny and terror who may come to the US to seek freedom
7. our collective and individual human rights
8. our collective and individual civil liberties
9. our collective and individual civic responsibilities
10. the union of 50 States
11. the individuation of 50 States within this union
12. To honor the men smart enough to draft our Constitution and Bill of Rights and the women who supported this cause behind the scenes.