website/static/playground.js/setEditorStyles
function setEditorStyles() {
  var options = getOptions();

  inputEditor.setOption("placeholder", getExample(options.parser));

  var mode = getCodemirrorMode(options);
  inputEditor.setOption("mode", mode);
  outputEditor.setOption("mode", mode);
  output2Editor.setOption("mode", mode);

  inputEditor.setOption("rulers", [
    { column: options.printWidth, color: "#eeeeee" }
  ]);

  [outputEditor, output2Editor].forEach(function(editor) {
    editor.setOption("rulers", [
      { column: options.printWidth, color: "#444444" }
    ]);
  });
  document.querySelector(".ast").style.display = options.ast ? "" : "none";
  document.querySelector(".doc").style.display = options.doc ? "" : "none";
  document.querySelector(".output2").style.display = options.output2
    ? ""
    : "none";
}