I like to mail from the Fastmail web interface. But I cannot set this as my default mail application anywhere.

The one where you did not need to enable Javascript to be able to launch your preferred mail program from a web page.

The problem that most people these days use webmail, and most don't have mailto: configured to open their webmail client, so clicking on a mailto: link results in the Outlook application that you never use taking two minutes to open.

I think it's nifty, and I'll probably implement it on my personal site.