In the box model, the width/height, padding, and border of the content area cannot be negative values, with the exception of margin, which can be negative values.
I don’t know much about the skills of using negative margin values. I will add more when I have the opportunity in the future. Here is a classic application. The negative value of margin-left is combined with floating to achieve a fluid layout that does not change the DOM structure.
This demo is the same as the one in the CSS float analysis article.