If you don't have "Enable Autosave" checked in your Preferences, you'll need to hover your mouse over the button with the three dots and then click "Save" before you close the Arduino Web Editor in your browser.
If you don't have "Enable Autosave" checked in your Preferences, you'll need to hover your mouse over the button with the three dots and then click "Save" before you close the Arduino Web Editor in your browser.