Labels inside a input - css

Question

How to put form labels inside a input form only on mobile?
example:
my code:
<form>
<div class="form-group">
<label class="form-control-label" for="firstName">Firstname</label>
<input type="text" class="form-control" id="firstName" name="firstName" placeholder="Firstname" required />
</div>
<div class="form-group">
<label class="form-control-label" for="lastName">Lastname</label>
<input type="text" class="form-control" id="lastName" name="lastName" placeholder="Lastname" required />
</div>
<div class="form-group">
<label class="form-control-label" for="phone_number">Mobile Number</label>
<input type="text" class="form-control" id="phone_number" name="phone_number" placeholder="A valid 9 or 10 digit phone number" maxlength="10"required pattern="0[689]?[0-9]{8}" />
</div>
</form>

Answer 1

Something like this? I changed the position of input and label in the HTML
.form-group {
position: relative;
min-height: 3.5em;
}
input.form-control {
height: 3em;
position: absolute;
top: 0;
left: 0;
}
label.form-control-label {
position: absolute;
font-size: .8em;
top: 0;
left: 5px;
text-transform: uppercase;
}
<form>
<div class="form-group">
<input type="text" class="form-control" id="firstName" name="firstName" placeholder="Firstname" required />
<label class="form-control-label" for="firstName">Firstname</label>
</div>
<div class="form-group">
<input type="text" class="form-control" id="lastName" name="lastName" placeholder="Lastname" required />
<label class="form-control-label" for="lastName">Lastname</label>
</div>
<div class="form-group">
<input type="text" class="form-control" id="phone_number" name="phone_number" placeholder="A valid 9 or 10 digit phone number" maxlength="10" required pattern="0[689]?[0-9]{8}" />
<label class="form-control-label" for="phone_number">Mobile Number</label>
</div>
</form>

Answer 2

You can use like the below method. We have to write mobile screen style with in the media queries
body{
font-family:arial;
}
.form-group{
position:relative;
}
.input-element{
padding:30px 5px 5px 5px;
width:100%;
border:1px solid #CCC;
}
#media screen and (max-width: 767px) {
.label-element{
position:absolute;
top:5px;
left:5px;
font-size:12px;
color:#666;
}
}
<div class="form-group"><label class="label-element">First Name</label>
<input type="text" class="input-element"/></div>

Answer 3

I've just added a border to form-group and overwritten some bootstrap code.
For mobile only, try to use CSS3 Media Queries.
You could try this:
.form-group {
border: 1px solid black; /* Change border width and color here */
}
/* the !important declaration is for this snippet, because this snippet include bootstrap after this declarations **/
.form-control {
border: none !important; /* No border for input */
box-shadow: none !important; /* No border for input */
}
.form-control:focus {
box-shadow: none !important; /* Remove the blue shining at focus */
}
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet"/>
<form>
<div class="form-group">
<label class="form-control-label" for="firstName">Firstname</label>
<input type="text" class="form-control" id="firstName" name="firstName" placeholder="Firstname" required />
</div>
<div class="form-group">
<label class="form-control-label" for="lastName">Lastname</label>
<input type="text" class="form-control" id="lastName" name="lastName" placeholder="Lastname" required />
</div>
<div class="form-group">
<label class="form-control-label" for="phone_number">Mobile Number</label>
<input type="text" class="form-control" id="phone_number" name="phone_number" placeholder="A valid 9 or 10 digit phone number" maxlength="10" required pattern="0[689]?[0-9]{8}" />
</div>
</form>

Answer 4

Inspired by another answer, here is a short way to do it:
.input-group {
position: relative;
}
.input-label {
position: absolute;
top: 5px;
left: 5px;
color: #666;
}
.input-element {
padding: 30px 5px 5px 5px;
}
<div class="input-group">
<label class="input-label">First name</label>
<input type="text" class="input-element" />
</div>