Is there an href attribute for skype, like "mailto:" or "tel:"?

You can use skype:<username|phonenumber>[?[add|call|chat|sendfile|userinfo]] or callto:<screenname|phonenumber>, according to this wikipedia page:

http://en.wikipedia.org/wiki/URI_scheme#callto:

If your mobile site is (generally) used by iOS devices, you can use :

<a href="skype:YourSkypeName?call">Call me on Skype</a>

Apple iOS devices automatically recognizes intents in link with Skype and FaceTime.