El Capitan’s Safari has a great new tool for web designers. In the Develop menu, there is now an Enter Responsive Design Mode menu item. (To turn on the Develop menu, go to Safari > Preferences, and check Show Develop menu in menu bar.) When you switch to this mode, you see the current web page in the following display.
You can view the web page easily as it displays on a range of iOS devices, as well as other browser resolutions, by clicking the icons above the page display. You can also choose the user agent (which web browser is being emulated), and choose to view your assets in 1x, 2x, or 3x.
If you design websites, this is a big time-saver. You no longer have to manually resize windows, or view sites on other devices.