XML elements are case sensitive. In HTML, <h1> and <H1> are the same; in XML, they're not. If you try to end an <h1> element with a </H1> tag, you'll get an error. In the example below, the heading at the top is illegal, while the one at the bottom is fine.

<!-- NOT legal XML markup -->
<h1>Elements are 
  case sensitive</H1>

<!-- legal XML markup -->
<h1>Elements are 
  case sensitive</h1>