title: Can I detect the user viewable area on the browser?
body: <p>As you can see the image below, there is "A", "B", "C", "D" and "E" on the website, and the user may only can see the A, B, and a little parts of D in their browser. They need to require to scroll down the browser or some users may have a bigger screen, or a longer window on their browser that allow they can even see the element C. </p>
  
  <p>Ok, my question is, is this possible to let me know what the user seeing on their browser using javascript? In this element, is "A", "B" and "D".</p>
  
  <p><img src="https://i.stack.imgur.com/Y4ob8.png" alt="enter image description here"></p>