This is meant to look like the blinky lights that were de rigueur on all computers in science fiction movies and TV shows from the ’60s until the late ’90s.
It uses the DOM and JavaScript (now ECMAScript) to create and refer to HTML table cells, and CSS to set the size and color of the cells.
It works on browsers that are up-to-date with those technologies, but not on older browsers. You can do something like this with older browsers, but the coding is a lot more messy.
You can make more than one panel in a page, and arrange the panels as you like. Instructions are in the source!