循環小数(2進)にならない浮動小数点用数値、固定小数点の精度(ビット幅)決定の参考値、などに。
See the Pen 0.5^n 表 by Ikiuo (@ikiuo) on CodePen.
<!DOCTYPE html>
<html lang="ja">
  <head>
    <meta charset="utf-8">
    <title>2^-n表</title>
    <style>
     table {
         border: solid 1px #e2e2e2;
         border-collaspe: collaspe;
         border-spacing: 0;
     }
     th, td {
         border: solid 1px #e2e2e2;
     }
     th { background: white; }
     td { padding: 2px 2px 2px 4px; }
     .eline { background: #edefef; }
     .oline { background: #f5f6f6; }
    </style>
  </head>
  <body>

    <script>

     function toClipboard(text) {
         navigator.clipboard.writeText(text).then(() => {
             // success
         }, () => {
             // failed
         });
     }

     function pow5n(n) {
         var a = 1n;
         var b = 5n;
         while (n) {
             if (n & 1)
                 a *= b;
             b *= b;
             n >>= 1;
         }
         return a;
     }

     function expstr(n) {
         const v = '' + pow5n(n);
         const s = v.length >= 9 ? v.slice(0, 3) + '⋅⋅⋅' + v.slice(-3) : v;
         return `10<sup>-${n - v.length}</sup> × 0.${s}`;
     }

     function fullstr(n) {
         const v = '' + pow5n(n);
         return '0'.repeat(n - v.length) + v;
     }

     window.onload = (() => {
         document.body.innerHTML = [
             '<table>',
             // '<tr><th colspan="3" style="font-size: xx-large;">0.5<sup>n</sup> 表</th></tr>',
             '<tr><th>n</th><th>指数形式(途中省略)</th><th>2<sup>-n</sup> 全桁</th></tr>',
             [...Array(64)].map((_, i) => {
                 const s = '0.' + fullstr(i+1);
                 return [
                     `<tr class="${i & 1 ? 'oline' : 'eline'}">`,
                     `<td style="text-align: right;">${i+1}<sup>&nbsp;</sup></td>`,
                     `<td onclick="toClipboard('${s}')">${expstr(i+1)}</td>`,
                     `<td onclick="toClipboard('${s}')">${s}</td>`,
                     '</tr>',
                 ].join('');
             }).join(''),
             '</table>',
         ].join('');
     });

    </script>
  </body>
</html>