Fiddler - Fiddler is a debugging program for websites.
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.
Code Cola - Code Cola is a chrome extension to editor online pages' style visibility.