portable text editor, copy and paste into address bar

min

data:text/html,<textarea contenteditable spellcheck=false ></textarea><button>Save</button><style>body{margin:0; padding:0;}textarea{background%3A%23666%3Bcolor%3A%230F0%3Bheight:100%;width:100%;line-height:1.3;font-size:14px;letter-spacing:1px;padding:10px;}button{position:absolute;color:black;right:20px;top:20px;border:1px solid grey;font-size:14px;padding:6px 14px;border-radius:8px;background%3A%239ECFF5%3B}</style><script>p = document.body.firstChild;p.focus();var val,tabs = 2,spaces = (function (s, t){while (--t) s += s;return s}(' ', tabs));p.onkeydown = function (e) {val = this.value;sstart = this.selectionStart;s = val.substr(0, sstart);n = val.substr(sstart);if (e.which === 9) {this.value = s + spaces + n;this.selectionStart = this.selectionEnd = sstart + tabs;return false;}};document.querySelector('button').onclick = function () {window.location = 'data:application/json,' + val;}</script>

src

data:text/html,
<textarea contenteditable spellcheck=false ></textarea>
<button>Save</button>
<style>
  body{margin:0;}
  textarea{
    background%3A%23666%3B
    color%3A%230F0%3B
    height:100%;
    width:100%;
    line-height:1.3;
    font-size:14px;
    letter-spacing:1px;
    padding:10px;
  }
  button{
    position:absolute;
    color:black;
    right:20px;
    top:20px;
    border:1px solid grey;
    font-size:14px;
    padding:6px 14px;
    border-radius:8px;
    background%3A%239ECFF5%3B
  }
</style>
<script>
  p = document.body.firstChild;
  p.focus();
  var val,
      tabs = 2,
      spaces = (function (s, t){
          while (--t) s += s;
          return s
      }(' ', tabs));
  p.onkeydown = function (e) {
      val = this.value;
      sstart = this.selectionStart;
      s = val.substr(0, sstart);
      n = val.substr(sstart);
      if (e.which === 9) {
          this.value = s + spaces + n;
          this.selectionStart = this.selectionEnd = sstart + tabs;
          return false;
      }
  };
  document.querySelector('button').onclick = function () {
      window.location = 'data:application/json;' + val;
  }
</script>

tested in FF7 only.
ugly DHTML FTL