spec
articles
总结：
Within each stacking context, the following layers are painted in back-to-front order:
- the background(background color 、background image) and borders of the element forming the stacking context.
- the child stacking contexts with negative stack levels (most negative first).
- the in-flow, non-inline-level, non-positioned descendants.
- the non-positioned floats.
- the in-flow, inline-level, non-positioned descendants, including inline tables and inline blocks.
- the child stacking contexts with stack level 0 and the positioned descendants with stack level 0.
- the child stacking contexts with positive stack levels (least positive first).
