At first glance, these are ordinary monochrome icons, but if you think about it, their quality will not deteriorate. Obvious profit of such icons: you can freely experiment with them and not worry about height and width, a compact view, and not a bunch of files in the icons folder, well, and probably a small size, although you can argue with that. There are some limiters in color, one color or gradient is possible (but I think with a converter of the drawn vector into js code, perhaps more colors)
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If you want to use these icons or create your own on a similar basis, click on the icon - the code of the selected icon appears at the bottom, it is inserted into: