Web Inspector is your command center, giving you quick and easy access to the richest set of development tools ever included in a web browser. With Web Inspector, you see Safari’s developer tools in a clean, unified interface designed to make developing web applications for OS X and iOS more efficient. In OS X El Capitan, the enhanced design provides a more intuitive experience, placing each of it's core functions in separate tabs.
Elements. View and inspect the elements that make up the DOM of a webpage. The rendered HTML is fully editable on the left and details about the webpage’s nodes, styles, and layers are available in the sidebar on the right.
Network. See a detailed list of all network requests made to get every webpage resource including documents, images, scripts, stylesheets, XHRs and more.
Resources. Find every resource of a webpage, including documents, images, scripts, stylesheets, XHRs and other resources that make up a webpage, in a list organized by type.
Storage. Find details about the data stored by a webpage such as cookies, local storage, session storage, application cache and index.