Why are there CSS rules using -9999px

Question

On the Twitter site, I found the following CSS rules:

.ProfileCanopy-headerBg img {
  position: absolute;
  left: 0;
  right: 0;
  top: -9999px;
  bottom: -9999px;
  margin: auto 0;
  width: 100%;
} 

What could be the reason for the limit -9999px?

Answer 1

It's a way to hide an element that still allows screen readers (and similar technologies) to read the contents, unlike if you were to use, say, display: none.

The value 9999px is arbitrary. Can be any negative number really but I guess it feels safer to really shove it far away :P