Mobile browsers don't support it. I've never been clear on the reasons, except maybe that there's a tension with the carriers over airwave bandwidth. Honestly

Mobile browsers don't support it. I've never been clear on the reasons, except maybe that there's a tension with the carriers over airwave bandwidth. Honestly: there's a Javascript interface in mobile Safari to access the location sensors and, IIRC the accelerometer, but the camera is offlimits. On the other hand, the HTML interface is a mime-multipart form and a file upload - it's only convention that says "upload images only" - and the mobile platforms are very particular about access to files on the device. iOS for sure won't allow one app to touch the files of another.