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So just this once I'm going to direct you to Reddit. They've tried to dissect it here: https://www.reddit.com/r/webdev/comments/b0wcrm/how_is_this_website_perfo

• devilduck 6y ago
  Oh you are just the most humble aren't you
• codethief 6y ago
  TL;DR Position an overlay element (in this case, a <canvas>) at the bottom of the browser viewport and draw some squares in the same color as your site's background color.
  • monokai_nl 6y ago
    Yes, correct. And this method only works because the background is the same color as the overlaid blocks and the font is monospace, so you can precisely position the blocks over the characters.