The challenge description hints to Finland. We’re given a flight booking website which lets us query for European capitals destinations.
For any other capital searched, the query returns a price in EUR. For Helsinki, it returns a price in px, which makes us think of pixels.
Also, the query for Helsinki is the only one which returns completed td class in the table HTML (classes start with “w”).
Therefore, looking in the bootstrap CSS file (get it here), we find an ‘if’ statement conditioned on the class starting with ‘w’ and browser width between 620px and 640px.
On resize, a QR code appears, which reveals the flag.