Development mode in toolbar

[wtf403]

toolbar in Arc when you open page on localhost:

[elyalon]

I tried making a zen mod for this and I believe it's not possible currently, because the current tab's URL is only accessible from the chrome's DOM by a text node (inside #urlbar-input), and it's not possible to create a CSS selector based on a text node, the current tab's URL needs to be exposed somewhere as an attribute (like data-url="<url>")