The margin-top property is functioning properly, while the top property is not having

When it comes to aligning my login page with CSS, I've encountered a problem distinguishing between the margin-top and top properties. While margin-top pushes the login page based on the first element, top defines its position. This has led me to avoid using margin-top in my media query codes, as unfortunately, top does not seem to be effective either.

In an attempt to utilize the top property in my CSS, I added the following:

#loginpage {
    position: absolute;
    width: 100%;
    top: -42%;
}

This implementation did not work for me.

The only successful method was adjusting the margin-top as follows:

margin-top: -20%;

Above my login page, there is a slideshow header and a Twitter Bootstrap navbar defined as:

#twitterbootstrap {
    position: relative;
}

#SlideshowHome {
    position: absolute;
    left: 0%;
    width: 100%;
    top: 6%;
    height: 20%;
}

This is how my login page is structured:

<div id="loginpage">
    <table id="loginpage1" width="100%">
        <tr>
            <td>
                <h2>Login</h2>
                <br />
            </td>
        </tr>

        <tr>
            <td>
                <b>Username:</b> <asp:TextBox ID="txtUsername" runat="server"></asp:TextBox>
                <br />
                <br />
            </td>
        </tr>

        <tr>
            <td>
                <b>Password:</b> <asp:TextBox ID="txtPassword" runat="server" TextMode="Password"></asp:TextBox>
            <br />
            <br />
            </td>
        </tr>
    </table>
</div>

Here are the details of the Twitter Bootstrap and Slideshow Header sections:

<div id="SlideshowHome">
    <img src="image/s1.jpg" name="slide" style="width: 100%; height: 150%">
</div>

<div id="twitterbootstrap">
    <div class="navbar navbar-inverse navbar-fixed-top">
        <div class="navbar-inner">
            <div class="container">
                <a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
                    <span class="icon-bar"></span>
                    <span class="icon-bar"></span>
                </a>
                <a class="brand" href="#">iPolice's Menu</a>
                <div class="nav-collapse">
                    <ul class="nav">
                        <li class="divider-vertical"></li>
                        <li><a href="Login.aspx">Login</a></li>
                        <li class="divider-vertical"></li>
                        <li><a href="RecoverPassword.aspx">Recover Password</a></li>
                        <li><a href="https://www.facebook.com/singaporepoliceforce?ref=ts&fref=ts"><img src="image/facebook.jpg" style="width: 100px; height: 30px"></a></li>
                        <li><a href="https://twitter.com/SingaporePolice"><img src="image/twitter.jpg" style="width: 100px; height: 30px"></a></li>
                    </ul>
                </div>
            </div>
        </div>
    </div>
</div>

Given these implementations, I'm curious why changing the margin-top affects the position while adjusting the top property doesn't seem to have any impact?

Answer №1

When using a value of top in percentage, it is based on the height of the containing block according to this documentation. However, when implementing margin-top in percentage, it is actually based on the WIDTH of the containing block as explained in this resource, leading to a discrepancy in size.

If your login page consists solely of absolutely positioned divs, then the body serving as the containing block may essentially be empty with no defined height, although its width remains set at 100% of the window. This explains why margin-top behaves differently from top.

To address this issue, it is recommended to explicitly assign a height to the container element.

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