<form action="#" method="post">

    <div class="input_block">
        <input type="text" name="name" id="name_input">
        <label for="name_input">
            <span>Full Name:</span>
        </label>
    </div>

    <div class="input_block">
        <input type="text" name="phone" id="phone_input">
        <label for="phone_input">
            <span>Phone Number:</span>
        </label>
    </div>

</form>

.input_block {
    margin: 0 0 25px 0;
    width: 344px;
    height: 66px;
    position: relative;
}

.input_block input {
    display: block;
    padding: 0;
    width: 344px;
    height: 64px;
    border: none;
    border-bottom: 2px solid #000;
    background: none;
    font-family: "ProximaNovaBold";
    font-size: 24px;
    color: #000;
    outline: none;
}

.input_block label {
    display: block;
    width: 344px;
    height: 64px;
    position: absolute;
    top: 0;
    left: 0;
}

.input_block label span {
    display: block;
    height: 24px;
    -webkit-transition: all 0.3s ease;
    -moz-transition: all 0.3s ease;
    -o-transition: all 0.3s ease;
    -ms-transition: all 0.3s ease;
    transition: all 0.3s ease;
    font-family: "ProximaNovaBold";
    font-size: 24px;
    line-height: 24px;
    color: #000;
    position: absolute;
    top: 21px;
    left: 0;
}

.input_block input:focus + label span {
    font-size: 14px;
    top: -10px;
}