animation in Outlook Classic

Before

<style>
  @keyframes pulse { 0%, 100% { opacity: 1; } 50% { opacity: 0.5; } }
  .badge { animation: pulse 2s infinite; }
</style>
<span class="badge">New!</span>

After

<!-- Use an animated GIF for the effect -->
<img src="https://example.com/badge-animated.gif"
  width="60" height="24" alt="New!"
  style="display: inline-block; border: 0;" />