What is the difference between the functionality of
.//input[@id=\'stack\'] and //input[@id=\'stack\']
.//input[@id=\'stack\']
//input[@id=\'stack\']
. always represents the current element. So your first statement looks for all <input id='stack'> beneath the actual element while the latter globally looks for all input elements with the id 'stack'.
.
<input id='stack'>