These are instructions for taking screenshots of an entire webpage, not just the part of the webpage visible in the browser.
This requires Safari 11.3, which comes on macOS 10.3.4.
- Open the website in Safari
- If needed, go Safari > Preferences > Advanced > Show Develop Menu in Menu Bar
- Go Develop > Enter Responsive Design Mode
- Click on the icons to choose your device and orientation.
- Go Develop > Show Web Inspector
- Right-click on the on purple "html" element showing up in the element tree, and select Capture Screenshot
- Open the website in Chrome
- In the menu bar, go to View > Developer > Developer Tools
- Click the meatball menu in the developer console and switch "Dock Side" to "Undock in separate window" to separate the browser window from the developer tools window.
- In the browser window, adjust the viewport size as desired. Be aware: Chrome cannot accurately simulate rendering on an iOS device. For that, you need to use Safari
- In the developer tools window, do CMD-SHIFT-P to open the "command window"
- Type "Capture full size screenshot"
No, it takes a screenshot of the entire page. If the page contains lazy loaded elements, or parts that appear only when in the visible area, make sure to scroll to bottom before capturing the screenshot. Attached an example of the output.