A favicon has very few pixels to communicate its shape. Begin with a bold silhouette or letterform, remove tiny text and interior detail, and test it at 16×16 and 32×32 pixels before polishing a large version.
Use the favicon generator to export the required files. Inspect the icon on light and dark browser chrome, in active and inactive tabs, and at normal display scale. Thin strokes, low contrast, and transparent edges often disappear at the size users actually see.
A recognizable small icon is better than a detailed one that only works enlarged.
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