bookmarklet

javascript:var s=document.createElement('script');s.src="https://gist.githack.com/wakaba/b90f120b2617afe5c2fc4d08d84e801f/raw/script.js";document.body.appendChild(s);void(0)

script.js

Array.prototype.forEach.call (document.querySelectorAll (".entry-title, h1, h2, h3, h4, h5, h6"), function (e) {
  var f = document.createElement ('dummy-container');
  while (e.firstChild) {
    f.appendChild (e.firstChild);
  }
  e.appendChild (f);
  var tryCount = 0;
  var maxTryCount = 30;
  var eWidth = e.offsetWidth;
  var minLastWidth = 3.5 * parseFloat (getComputedStyle (e).fontSize);
  e.style.transformOrigin = "left top";
  e.style.boxSizing = "border-box";
  var factor = 1;
  var rects = f.getClientRects ();
  while (rects.length > 1 && rects[rects.length-1].width < minLastWidth && tryCount++ < maxTryCount) {
    factor *= 0.99;
    e.style.transform = "scalex("+factor+")";
    e.style.width = eWidth / factor + "px";
    e.style.background = "red";
    e.setAttribute ('data-try-count', tryCount);
    rects = f.getClientRects ();
  }
});

bukkenfan.jp でブックマークレット実行

before

after

なるほど!!!