Experimental testing

HTML

<script src="https://fonts.googleapis.com/css?family=Open+Sans|Roboto+Mono"></script>
<button id="button">Stop</button>
<h1>Stats</h1>
<pre id="res">Raw: <span id="success">0</span> out of <span id="trials">0</span>
Rate: <span id="pct">0</span>%</pre>
<h1>Successes</h1>
<pre id="valid"></pre>

CSS

body {
  padding-left: 1em;
}

h1 {
  margin: 0;
  font-family: 'Open Sans';
}

pre {
  margin: 0;
  font-family: 'Roboto Mono', monospace;
}

#res {
  font-size: 3em;
}

button {
  font-size: 2em;
}

JavaScript

alert = x => {
  try {
    new Function(x);  // Throws `SyntaxError` if invalid syntax
    success++;        // If no error is thrown, increment
    spans.success.textContent = success;
    spans.valid.textContent = x + '\n' + spans.valid.textContent;
  } catch (e) { }
  spans.trials.textContent = i;
  spans.pct.textContent = (success / i * 100).toFixed(2);
};

var success, i, spans = {
  success: document.getElementById('success'),
  trials: document.getElementById('trials'),
  pct: document.getElementById('pct'),
  valid: document.getElementById('valid')
}, intervalId = null, button = document.getElementById('button');

success = i = 0;

function start() {
  if (intervalId === null) {
    intervalId = setInterval(_ => {
      i++;
      a="a=q;a=a.replace('q',uneval(a));for(b=c='a';d=a.split(c),c=d[Math.random()*~-d.length+1|0][0];b+=c);alert(b)";a=a.replace('q',uneval(a));for(b=c='a';d=a.split(c),c=d[Math.random()*~-d.length+1|0][0];b+=c);alert(b)
    // Above from http://codegolf.stackexchange.com/a/98146/48878
    }, 1);
  }
  button.onclick = stop;
  button.textContent = 'Stop';
}

function stop() {
  if (intervalId !== null) {
    clearInterval(intervalId);
    intervalId = null;
  }
  button.onclick = start;
  button.textContent = 'Start';
}

start();