title: Implement an input with a mask
body: <p>I would like to implement a mask for a text <code>input</code> field which accepts a date. The masked value should display directly inside of the <code>input</code>.</p>
  
  <p>Something like this:</p>
  
  <pre><code>&lt;input type='text' value='____/__/__'&gt;
  </code></pre>
  
  <p>I wrote the mask as a value in that example, but my intent is to allow people to write a date without typing <code>/</code> or <code>-</code> to separate months, years and days. The user should be able to enter numbers into the displayed field, while the mask enforces the format automatically as the user types.</p>
  
  <p>I have seen this behavior on other sites, but I have no idea how it works or how to implement it myself.</p>