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The file formats that are most appropriate for photos are JPG and WebP, and the formats appropriate for line art are PNG, SVG, and GIF. For JPG, due to high numbers of colors and levels of details, sizes of files are excessively high for web applications. In order to reduce load, image compressions are needed, even if the JPG compressions are lossy. Fortunately, the image is still in a rectangular shape. PNG, meanwhile, has an alpha channel feature to enforce transparency functions. The color mode is usually either RGB or index. Unfortunately, the photo becomes pixelated due to PNG compression.
Favicons, typically saved in .ICO, .SVG, .PNG, and .GIF formats, are used to mark bookmark icon. The browser does that by first saving the icon file in various file formats for various devices and browsers. When asked by a user, the favicon is saved into its bookmark manager.