How to alter a form element's properties before rendering?
In hook_template_preprocess_form_element the (input-)tags is already rendered.
You can use 

function hook/template_preprocess_input(&amp;$variables) {
  $variables['attributes']['title'] = 'my-title';
  $variables['attributes']['placeholder'] = 'my-placeholder';
}


to alter the input tag. use the same method for all other desired tags.
Here's a nice tutorial.


  You could make use of hook/template_preprocess to go a more generic
  way.