I hate to point to Google, but they have a format that allows prompting in Chrome whether to associate mailto to that webapp. Microsoft/Outlook.com/Hotmail does

I hate to point to Google, but they have a format that allows prompting in Chrome whether to associate mailto to that webapp. Microsoft/Outlook.com/Hotmail doesn't prompt for it (and apparently there's no way short of JS-whipping that will just accept mailto if you want to do it manually), but I have seen Yahoo prompting for that.