The answers on this question are not only wrong, but dangerous. CTRL+ALT+SHIFT+B will not indent HTML but XML. Consider the following HTML code:
<span class="myClass"></span>
The function 'Notepad++ -> Plugins -> XmlTools -> Pretty print (Xml only with line breaks)' (CTRL+ALT+SHIFT+B) will transform this to:
<span class="myClass"/>
which will not be displayed correctly anymore by your browser! I strongly advice against using this function to indent HTML.
Instead use the plugin Tidy2. This will indent the HTML correctly without bad side-effects (but it will also create <html>, <head>, <body>, ...
elements around your code, if these are not there).