If you need to capture an entire web page as an image, it can be difficult to do so because macOS screenshots don't allow you to capture all of the contents of a scolling window. However, you can use many different techniques to get the entire contents of a web page. Safari has a hidden feature to do this, but sometimes it won't work on certain pages. You can buy third-party pro capture software that will let you take perfect screen grabs of entire pages. You can also use a feature of the free Firefox browser, though it sometimes doesn't work perfectly.