所以我需要一个覆盖其整个父级的 div。（需要位于事件监听的前面，因此z-index需要高于父级。）如果父级没有固定大小，我该怎么办？例如： https: //jsfiddle.net/CypherK/hdgko1L4/<div class="container">    <div class="content"><!-- should be however big the container is-->        <div class = "mask"></div> <!-- should be in front of the entire content -->    </div></div>.container {  height: 3em;  width: 50%; /*could be anything*/  border: 1px solid green;}.content {  height: 1em;  width: 100% - 6px; /*intention: as big as container, minus own borders*/  border: 3px dashed blue;}.content .mask {  position: absolute;  z-index: 99;  height: calc(100% + 6px); /*intention: as big as content, including its borders*/  width: calc(100% + 6px); /*intention: as big as content, including its borders*/  margin: -3px; /*intention: place on border of parent*/  border: 2px dotted red;}