How to achieve centered layout in CSS layout

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        display: table-cell; /* Transform into a table */
        text-align: center; /* horizontal */
        vertical-align: middle; /* vertical */
    }
    #child {
        background-color: blue;
        color: white;

        display: inline; /* Child elements are set to inline or inline block*/
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child">Content</div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        position: relative; /* Set relative positioning */
    }
    #child {
        height: 50px;
        width: 50px;
        color: white;
        background-color: blue;

        /* Absolute positioning, after setting the 4 directions to 0, the margin is set to auto */
        position: absolute;
        left: 0;
        top: 0;
        right: 0;
        bottom: 0;
        margin: auto;
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child"></div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        display: flex; /* Convert the parent element to a flexible box*/
        display: -webkit-flex; /* Safari */
    }
    #child {
        height: 50px;
        width: 50px;
        background-color: blue;

        margin: auto;
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child"></div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        position: relative; /* Set relative positioning */
    }
    #child { /* When the child element knows its own width and height*/
        background-color: blue;

        width: 50px;
        height: 50px;
        margin-top: -25px;
        margin-left: -25px;
        position: absolute;
        left: 50%;
        top: 50%;
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child"></div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        position: relative; /* Set relative positioning */
    }
    #child { /* Child element does not know its own width and height, use transform's translate */
        border: 1px solid blue;

        position: absolute;
        top: 50%;
        left: 50%;
        -webkit-transform: translate(-50%,-50%);
        -ms-transform: translate(-50%,-50%);
        -o-transform: translate(-50%,-50%);
        transform: translate(-50%,-50%);
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child">
        <div id="content">
            Content 1
            <br/>
            Content 2
        </div>
    </div>
</div>

<!-- css -->
<style>
    #parent {
        height: 200px;
        width: 200px;
        border: 1px solid red;

        display: flex; /* Convert the parent element to a flexible box*/
        display: -webkit-flex; /* Safari */
        justify-content: center;/* horizontal */
        align-items: center; /* vertical */
    }
    #child {
        height: 50px;
        width: 50px;
        background-color: blue;
    }
</style>

<!-- html -->
<div id="parent">
    <div id="child"></div>
</div>