|In addition to the many ways of setting file-type association, Notepad++ offers to include the context menu button when it installs. That puts an "Edit with Notepad++" option when you right-click files.|
What do you mean it doesn't work with a HTML 5 document? You link to it just as you did in previous versions of (x)HTML.
<link rel="stylesheet" type="text/css" href="style.css" media="all">
Apologies if I don't respond to your reply immediately. I don't check this site daily, but you're welcome to PM me as a reminder.