Jquery UI tooltip does not support html content

Question

Edit: Since this turned out to be a popular answer, I'm adding the disclaimer that @crush mentioned in a comment below. If you use this work around, be aware that you're opening yourself up for an XSS vulnerability. Only use this solution if you know what you're doing and can be certain of the HTML content in the attribute.

---

The easiest way to do this is to supply a function to the content option that overrides the default behavior:

$(function () {
      $(document).tooltip({
          content: function () {
              return $(this).prop('title');
          }
      });
  });

Example: http://jsfiddle.net/Aa5nK/12/

Another option would be to override the tooltip widget with your own that changes the content option:

$.widget("ui.tooltip", $.ui.tooltip, {
    options: {
        content: function () {
            return $(this).prop('title');
        }
    }
});

Now, every time you call .tooltip, HTML content will be returned.

Example: http://jsfiddle.net/Aa5nK/14/

Instead of this:

$(document).tooltip({
    content: function () {
        return $(this).prop('title');
    }
});

use this for better performance

$(selector).tooltip({
    content: function () {
        return this.getAttribute("title");
    },
});

I solved it with a custom data tag, because a title attribute is required anyway.

$("[data-tooltip]").each(function(i, e) {
    var tag = $(e);
    if (tag.is("[title]") === false) {
        tag.attr("title", "");
    }
});

$(document).tooltip({
    items: "[data-tooltip]",
    content: function () {
        return $(this).attr("data-tooltip");
    }
});

Like this it is html conform and the tooltips are only shown for wanted tags.

You can also achieve this completely without jQueryUI by using CSS styles. See the snippet below:

div#Tooltip_Text_container {
  max-width: 25em;
  height: auto;
  display: inline;
  position: relative;
}

div#Tooltip_Text_container a {
  text-decoration: none;
  color: black;
  cursor: default;
  font-weight: normal;
}

div#Tooltip_Text_container a span.tooltips {
  visibility: hidden;
  opacity: 0;
  transition: visibility 0s linear 0.2s, opacity 0.2s linear;
  position: absolute;
  left: 10px;
  top: 18px;
  width: 30em;
  border: 1px solid #404040;
  padding: 0.2em 0.5em;
  cursor: default;
  line-height: 140%;
  font-size: 12px;
  font-family: 'Segoe UI';
  -moz-border-radius: 3px;
  -webkit-border-radius: 3px;
  border-radius: 3px;
  -moz-box-shadow: 7px 7px 5px -5px #666;
  -webkit-box-shadow: 7px 7px 5px -5px #666;
  box-shadow: 7px 7px 5px -5px #666;
  background: #E4E5F0  repeat-x;
}

div#Tooltip_Text_container:hover a span.tooltips {
  visibility: visible;
  opacity: 1;
  transition-delay: 0.2s;
}

div#Tooltip_Text_container img {
  left: -10px;
}

div#Tooltip_Text_container:hover a span.tooltips {
  visibility: visible;
  opacity: 1;
  transition-delay: 0.2s;
}

<div id="Tooltip_Text_container">
  <span><b>Tooltip headline</b></span>
  <a href="#">
    <span class="tooltips">
        <b>This is&nbsp;</b> a tooltip<br/>
        <b>This is&nbsp;</b> another tooltip<br/>
    </span>
  </a>
  <br/>Move the mousepointer to the tooltip headline above. 
</div>

The first span is for the displayed text, the second span for the hidden text, which is shown when you hover over it.

To expand on @Andrew Whitaker's answer above, you can convert your tooltip to html entities within the title tag so as to avoid putting raw html directly in your attributes:

$('div').tooltip({
    content: function () {
        return $(this).prop('title');
    }
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script src="http://code.jquery.com/ui/1.9.2/jquery-ui.js"></script>
<div class="tooltip" title="&lt;div&gt;check out these kool &lt;i&gt;italics&lt;/i&gt; and this &lt;span style=&quot;color:red&quot;&gt;red text&lt;/span&gt;&lt;/div&gt;">Hover Here</div>

More often than not, the tooltip is stored in a php variable anyway so you'd only need:

<div title="<?php echo htmlentities($tooltip); ?>">Hover Here</div>