How to feature detect from script

[bokand]

Previously we had script-based detection by checking for allow in <a> but #5 says allow is probably not the right keyword. I'm currently leaning to replacing allow with a new value for rel but that's no longer feature detectable from script.

Some potential options:

• Something specific for feature detection, e.g. 'multilink' in navigator
• Instead of a rel value add a new attribute, e.g. <a multi> which can be feature detected