CSS Input with width: 100% goes outside parent’s bound

    #mainContainer {
        line-height: 20px;
        font-family: "Helvetica Neue",Helvetica,Arial,sans-serif;
        background-color: rgba(0,50,94,0.2);
        margin: 20px auto;
        display: table;
        -moz-border-radius: 15px;
        border-style: solid;
        border-color: rgb(40, 40, 40);
        border-radius: 2px 5px 2px 5px / 5px 2px 5px 2px;
        border-radius: 2px;
        border-radius: 2px 5px / 5px;
        box-shadow: 0 5px 10px 5px rgba(0,0,0,0.2);
    }
    
    .loginForm {
        width: 320px;
        height: 250px;
        padding: 10px 15px 25px 15px;
        overflow: hidden;
    }
    
    .login-fields > .login-bottom input#login-button_normal {
        float: right;
        padding: 2px 25px;
        cursor: pointer;
        margin-left: 10px;
    }
    
    .login-fields > .login-bottom input#login-remember {
        float: left;
        margin-right: 3px;
    }
    
    .spacer {
        padding-bottom: 10px;
    }
    
    /* ELEMENT OF INTEREST HERE! */
    input[type=text],
    input[type=password] {
        width: 100%;
        height: 20px;
        padding: 5px 10px;
        background-color: rgb(215, 215, 215);
        line-height: 20px;
        font-size: 12px;
        color: rgb(136, 136, 136);
        border-radius: 2px 2px 2px 2px;
        border: 1px solid rgb(114, 114, 114);
        box-shadow: 0 1px 0 rgba(24, 24, 24,0.1);
    }
    
    input[type=text]:hover,
    input[type=password]:hover,
    label:hover ~ input[type=text],
    label:hover ~ input[type=password] {
        background:rgb(242, 242, 242) !important;
    }
    
    input[type=submit]:hover {
      box-shadow:
        inset 0 1px 0 rgba(255,255,255,0.3),
        inset 0 -10px 10px rgba(255,255,255,0.1);
    }

    <div id="mainContainer">
        <div id="login" class="loginForm">
            <div class="login-top">
            </div>
            <form class="login-fields" onsubmit="alert('test'); return false;">
                <div id="login-email" class="login-field">
                    <label for="email" style="-moz-user-select: none;-webkit-user-select: none;" onselectstart="return false;">E-mail address</label>
                    <span><input name="email" id="email" type="text"></input></span>
                </div>
                <div class="spacer"></div>
                <div id="login-password" class="login-field">
                    <label for="password" style="-moz-user-select: none;-webkit-user-select: none;" onselectstart="return false;">Password</label>
                    <span><input name="password" id="password" type="password"></input></span>
                </div>
                <div class="login-bottom">
                    <input type="checkbox" name="remember" id="login-remember"></input>
                    <label for="login-remember" style="-moz-user-select: none;-webkit-user-select: none;" onselectstart="return false;">Remember my email</label>
                    <input type="submit" name="login-button" id="login-button_normal" style="cursor: pointer" value="Log in"></input>
                </div>
            </form>
        </div>
    </div>