input-filter.js

function setInputFilter(textbox, inputFilter, errMsg) {
  [
    "input",
    "keydown",
    "keyup",
    "mousedown",
    "mouseup",
    "select",
    "contextmenu",
    "drop",
    "focusout",
  ].forEach(function (event) {
    textbox.addEventListener(event, function (e) {
      if (inputFilter(this.value)) {
        // Accepted value.
        if (["keydown", "mousedown", "focusout"].indexOf(e.type) >= 0) {
          this.classList.remove("input-error");
          this.setCustomValidity("");
        }

        this.oldValue = this.value;
        this.oldSelectionStart = this.selectionStart;
        this.oldSelectionEnd = this.selectionEnd;
      } else if (this.hasOwnProperty("oldValue")) {
        // Rejected value: restore the previous one.
        this.classList.add("input-error");
        this.setCustomValidity(errMsg);
        this.reportValidity();
        this.value = this.oldValue;
        this.setSelectionRange(this.oldSelectionStart, this.oldSelectionEnd);
      } else {
        // Rejected value: nothing to restore.
        this.value = "";
      }
    });
  });
}

setInputFilter(
  document.getElementById("font-size"),
  function (value) {
    return /^\d*\.?\d*$/.test(value); // Allow digits and '.' only, using a RegExp.
  },
  "Only digits and '.' are allowed"
);