There are three kinds:
1. The parent tag's position attribute is relative;
2. The question tag has no position attribute (excluding static);
3. The question tag has a float attribute.
It is also easy to understand why the z-index of the insert will fail after the parent sets the position and z-index. There are three solutions:
1. Change position:relative to position:absolute;
2. Add position attribute to floating elements (such as relative, absolute, etc.);
3. Remove floating.