A browser is a general-purpose application on a general-purpose computer, so it needs three things settled before it can be left alone: hide the browser’s own interface, bring it back after a restart, and stop the device powering down.
Go full screen
The browser’s toolbars and tabs should not be visible on a screen people are looking at.
Press F11 on Windows or Linux, or Control + Command + F on macOS.
Start automatically in kiosk mode
Full screen by hand is fine for a demonstration. A screen that runs on its own needs the browser to open the page by itself after a power cut.
Set the browser to launch at startup, open anywhere.screenlyapp.com, and run in kiosk mode. Kiosk mode removes the address bar and the controls that let a passer-by browse somewhere else. Chrome, Edge, and other Chromium browsers support it through a command-line flag on the shortcut that starts them. Safari has no kiosk flag, so on a Mac use full screen instead.
Stop the device sleeping
The most common cause of a blank browser screen is the device’s own power settings rather than Screenly.
Turn off sleep in the device’s power settings.
Turn off the screensaver.
Remove any scheduled power-off.
On a laptop, check the settings that apply when the lid is closed and when it is running on mains power, since those are configured separately.
Test a restart before the device is mounted somewhere awkward. Pull the power, plug it back in, and confirm the screen returns to your content without anyone touching a keyboard.