Use JavaScript to set the favicon on a page so that it cycles through the characters (code points) in a string in similar way to how a marquee would.

marquee-favicon.js

/**
 * Sets the favicon for the page to show one letter per second of the
 * MARQUEE_TEXT constant.  The background will be a solid color that slowly
 * changes, cycling through different hues.
 */
addEventListener('DOMContentLoaded', () => {
  const MARQUEE_TEXT = 'A TEST ';

  // The canvas that will be used to generate the image that will show as the
  // favicon once every second.
  const size = 64;
  const canvas = document.createElement('canvas');
  canvas.width = canvas.height = size;
  const ctx = canvas.getContext('2d');

  // Inject favicon
  const link = document.createElement('link');
  link.rel = 'icon';
  link.type = 'image/png';

  // Remove all `<link>` elements that are for icons and then just add our
  // favicon `<link>` element.
  document.querySelectorAll("link[rel~='icon']").forEach(el => el.remove());
  document.head.appendChild(link);

  // The array of characters to show.
  // NOTE:
  //   Using Array.from() instead of String#.split() so that code points will be
  //   split correctly.
  let marquee = Array.from(MARQUEE_TEXT);
  
  // Update the favicon once every second.
  setInterval(() => {
    // Draw triangle with horizontal hypotenuse (bottom edge)
    ctx.fillStyle = `hsl(${new Date().getSeconds() * 12}deg, 100%, 30%)`;
    ctx.beginPath();
    ctx.moveTo(0, 0);       // Top-left (right angle)
    ctx.lineTo(0, size);      // Top-right
    ctx.lineTo(size, size);     // Bottom-right (hypotenuse edge)
    ctx.lineTo(size, 0);     // Bottom-right (hypotenuse edge)
    ctx.closePath();
    ctx.fill();

    // Draw one letter at a time of the marquee.
    let [first, ...rest] = marquee;
    marquee = [...rest, first];
    ctx.fillStyle = 'white';
    ctx.font = `bold ${size * 7 / 8}px sans-serif`;
    ctx.textAlign = 'center';
    ctx.textBaseline = 'middle';
    ctx.fillText(first, size / 2, size / 2);
    
    // Update the favicon.
    link.href = canvas.toDataURL('image/png');
  }, 1e3);
});