First of all placeholder is not a valid attrinbute for a label - it's for inputs only. You could change this to a data attribute, but I don't really see the point in this, you might as well just use the label as follows (alt is also not a valid attribute
<label for="last_name">Last name</label>
You'll then need to adjust the positioning etc. of this label.
I would also consider adding a class to a wrapper for the label/input combination for better control and to avoid said specificity issues. I've done a quick example using BEM naming conventions using sass as a preprocessor. This isn't meant to replicate the behavior of your code, but should act as a good starting point.