css3: translateX to center element horizontally

Question

I\'m trying to center horizontally an element: jsfiddle

On the child I

Answer 1

If you don't want to set the width of child-element

#child {
    display: flex;
    justify-content: center;
}

Answer 2

You were missing a position relative on your parent, position absolute on the child and the left and top values to help offset the child.

DEMO http://jsfiddle.net/nA355/6/

#parent {
    width: 300px;
    height: 100px;
    background-color: black;
    border: 1px solid grey;
    position: relative;
}
#child {
    position: absolute;
    height: 100px;
    width: 20px;
    left: 50%;
    top: 50%;
    -webkit-transform: translateX(-50%) translateY(-50%);
    -moz-transform: translateX(-50%) translateY(-50%);
    transform: translateX(-50%) translateY(-50%);
    background-color:red;
}

Answer 3

You can do it by display flex and justify-content is center.

Example: http://jsfiddle.net/ugw9o3qp/1/

#parent {
    display: flex;
    justify-content: center;
    width: 300px;
    height: 100px;
    background-color: black;
    border: 1px solid grey;
}
#child {
    height: 100px;
    width: 20px;
    background-color:red;
}

Answer 4

I use this technique to horizontally align some elements in situations where "text-align:center;" does not work correctly.

 position:relative; 
 left:50%;
 transform:translateX(-??em) /*or -??px or other unit of measure */

... "??" has to be tweaked depending on how wide the element is. I know it's kinda hacky, but seems to work on the screens I've tried it on so far.

In your fiddle, applying this technique, I get:

#parent {
    width: 300px
    height: 100px;
    background-color: black;
    border: 1px solid grey;
}
#child {
    position:relative;
    height: 100px;
    width: 20px;
    left:50%;
    transform: translateX(-10px); /*or  transform: translateX(-50%);*/
    -webkit-transform: translateX(-10px);/*or  -webkit-transform: translateX(-50%); */
    -ms-transform: translateX(-10px);/*or -ms-transform: translateX(-50%);*/
    background-color:red;
}

which exactly centers the child div horizontally.