The devtool is a set of web developer tools embedded in every browser

About

Every browser comes with a set of developer tools that permits

• to inspect the Dom
• to inspect the network request such as the HTTP request
• to inspect the cookie
• to execute javascript (ie the console)
• to test your CSS selector
• to measure the page load and rendering performance
• and much more

Access

Pressing F12 in most of the browser will bring you to the devtool.

Otherwise:

• Chrome > View > Developer > Developer Tools. See https://umaar.com/dev-tips/
• Firefox > Tools > Web Developer. (the old Firebug plugin is deprecated)