High lite a text on Specific X&Y Position in Javascript

Question

 Hello folks,
                I have an issue that I want to High lite a specific  text according to  its X And Y position in browser. for Example 
 <div>
<h4> 
Hi, This is your Invoice No 1234567890
</h4>
</div> 
This Text is under<div> <h4> tag. If i want to High lite  dynamically a specific X and Y position of <h4> then how it is possible using pure javascript.Please note  that need only in Javascript not in Jquery. I am not using any-kind of jquery.i f you have any code or idea to do this  then i am really appreciated.
Thank in Advance
KP Singh Chundawat