how to cancel opacity for a child element?

Answer 1

if you have parent background color - use RGBA, if you have parent image - use additional RGBA layer between parent and child divs playing with css position.

Answer 2

The opacity of the child will always be the opacity of the parent if the opacity of the child is 1.

This is not a problem with inheritance, but rather with the way opacity is calculated.

For instance,

<div id="parent">
    <div></div>
</div>

<div id="original">
</div>

<div id="quarter">
</div>

#parent div, #quarter {
    width: 100px;
    height: 100px;
    background-color: orange;
}

#parent div {
    opacity: 0.5;
}

#parent {
    opacity: 0.5;
}

#quarter {
    opacity: 0.25;
}

#quarter's opacity, from your perspective, is the same as that of #parent div, but in actual fact, #parent div has twice the opacity of #quarter. See this jsfiddle for more detail: http://jsfiddle.net/HUaNm/

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The only way to avoid this is to move the child out of the parent. Alternatively, depending on what you want here, you can also use rgba colors for the background/border/font color of the parent instead of opacity, but the effect is not the same as applying opacity.