The data fed to the applications I work on will come in the form of JSON, be it from an API or a static file.
To debug any problems that may arise I've become good friends with Chrome's DevTools and Charles Proxy. I know many of the browser specific quirks and how to make a website work cross-browser and -device.
In recent years I mainly worked with Ruby on Rails applications, but I also have experience with the PHP/MySQL stack.
Working side-by-side with designers, Adobe's Photoshop, InDesign and Illustrator are common tools to me.