Kaspar Loog discusses the challenges of website testing across different browsers and devices. There are many variables like operating system versions, device variations, resolutions and orientations, and browser versions that make thorough cross-browser testing difficult. Research shows that on average, testers only find about 30% of visual issues, while automated tools using computer vision can find closer to 70% of issues. While automated tools are faster and can cover more browsers and URLs, they still produce some false positives and negatives, so the best approach is a combination of human and automated testing for comprehensive cross-browser testing.