As XAML is XML, you need to use lang="XML". The only problem is how to show XML in HTML. Look at this solution using "Improve solution" (please don't commit!) or your browser's "View Page Source". You will get the idea.
If you write the code:
<pre lang="XML">
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<!-- ... -->
</Window>
</pre>
It will render as:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
</Window>
—SA