Hard to think of a good usecase for such a behaviour, as it is blocking some of the users input.
An easy way would be to use input::after
but this is not supported by any browser right now (thanks @JukkaK.Korpela).
But you can use a wrapper element and a data attribute, as follows:
<div class="placeholder" data-placeholder="my placeholder">
<input value="My text" />
</div>
With this css:
.placeholder
{
position: relative;
}
.placeholder::after
{
position: absolute;
left: 5px;
top: 3px;
content: attr(data-placeholder);
pointer-events: none;
opacity: 0.6;
}
Resulting in: 
Click here for jsFiddle demo.
Since you will have to do a lot of tweaking to make this look good, you may also consider using the wrapping <div>
element as a input "look alike":
<div class="editable" data-placeholder="my placeholder">
<input type="text" value="my Text" />
</div>
CSS:
.editable
{
position: relative;
border: 1px solid gray;
padding: 3px;
background-color: white;
box-shadow: rgba(0,0,0,0.4) 2px 2px 2px inset;
}
.editable > input
{
position: relative;
z-index: 1;
border: none;
background-color: transparent;
box-shadow: none;
width: 100%;
}
.editable::after
{
position: absolute;
left: 4px;
top: 5px;
content: attr(data-placeholder);
pointer-events: none;
opacity: 0.5;
z-index: 1;
}
Click here for the Demo 3. (with mocked <input />
)
Click here for the Demo 2. (with contenteditable)