dustlink.js - The longer you do not click, the darker the background-color will be.

dustlink.js

// dustlink.js
// Example HTML: <a class="DUSTLINK" href="https://example.com">example</a>
$(function() {
  var ds = localStorage.getItem('dusthash');
  if (!ds) {
    ds = '{}';
  }
  var dusthash = JSON.parse(ds);
  $(".DUSTLINK").on('click', function() {
    var key = $(this).attr('href');
    dusthash[key] = Date.now();
    ds = JSON.stringify(dusthash);
    localStorage.setItem('dusthash', ds);
  });
  $(".DUSTLINK").each(function() {
    var key = $(this).attr('href');
    var when = dusthash[key];
    var days = (Date.now() - when) / (24 * 60 * 60 * 1000);
    var full_days = 7; // 7 days = 50% darkness
    var light = 50 + (1.0 - days / full_days) * 50;
    var background_color = 'hsl(0, 0%, ' + light + '%)';
    var color = light < 75 ? 'white' : 'black';
    if (when) {
      $(this).css('color', color);
      $(this).css('background-color', background_color);
    } else {
      // Never clicked yet.
      $(this).css('color', '#800');
      $(this).css('background-color', 'hsl(0, 0%, 75%)');
    }
  });
});

cf. https://snap.textfile.org/20190115114836/